I'm sure you will already know a great deal of what's below, Speed, but I will try to explain these concepts in a more general sense so as perhaps to be of use to others reading here also. I am equally sure that Justin will disagree or object to some of what of I say. I am far from an expert but here is my understanding of Dasein (and angst). I have tried to keep the tone as conversational as possible. I've included lots of examples and have added some sneaky drawings I did to attempt to help explain things.
To understand the concept of Dasein, one must first understand the question that led to its formulation.
Heidegger is concerned with Being. Being is the fundamental question of his work. He distinguishes between two kinds of being: ontic and ontological. One of the problems of Being and Time is that, as well as introducing a startling array of new terminology, it uses pre-existing terms without defining them, assuming an accumulated specialized knowledge in the reader. One such term is 'ontic' being. I cursed not knowing what it meant when I first read the text. It's absolutely crucial to have a crystal-clear understanding of each of these terms before progressing to the concept of Dasein.
Ontic Being: Ontic being describes the physical properties of something's existence. It describes its factual being. Imagine you are back at High School and, sitting in class, the teacher asks you to describe carefully the contents of a test-tube. The description you would provide would correspond to an ontic analysis of the contents of the tube. An ontic description of a book might be: it has a red cover, a black spine and is quite heavy.
Ontological Being Ontological being describes the nature of what something IS. It dispenses with an analysis of appearance or physical description to understand the WHAT of a thing’s Being. A comb is ontically blue, with thirty teeth, light and made of plastic. A comb is ontologically FOR parting hair. Tidying hair is one facet of a combs' ontological being. The ontical being of a saucepan is a black, hollowed-out object with a wooden handle; one facet of its ontological being is cookery. 'Purpose' is perhaps too tendentious a term, but ontological Being certainly attempts to understand the primal 'nature' of a thing's Being, whereas ontical being understands its appearance.
The different types of being are differentiated in text in several ways. Some translators italicize ontological being; I use a capital letter. (‘being’ = ontic being) (‘Being’ = ontological being). I hope the above is clear. If not, please say so and perhaps I or someone else can explain further.
Now, Heidegger argues that the entire history of Western thought - reason, science, philosophy etc - concerns itself ONLY with ontic being. That is, with physical description. In arguably the most iconoclastic reconstitution of thought in the history of philosophy, he dismisses the entire body science and philosophy as wholly failing to engage with the concept of ontological Being. When science asks what an object IS, he argues, it only describes its appearance and properties.
Having thus distinguished the concepts of being and Being, Heidegger sees a monumental task before him. He wishes to understand the MEANING of ontological Being.
He makes some opening observations. Most existing objects, he realizes, are unable to understand their own ontological being. He calls such items ‘ontic items.’ A pencil, a can of soda, a hammer, a Slayer cd and a keyboard are all examples of ontic items. Man, however, is capable not only of understanding his existence (the ontic human body) in an ontological sense (through his awareness of himself in consciousness) but is also capable of understanding the Being of other humans and the Being of ontic items. Man is self-aware. This special distinction of ontological being and ontological awareness in humanity, Heidegger terms ‘Dasein.’ Dasein is the ontological being of man.
It is crucial to understand that Dasein is not a subject but a collective window of potential Being. Within the human World, a comb is most often ontologically ‘brushing-hair.’ Dasein is ALL the different potential Beings of humanity. In Being and Time Heidegger attempts to analyze the Being of Dasein.
(He states that he will cover the Being of ontic items later. Some argue that this remains the unfulfilled promise of his philosophy, others propose that he came to understand that Dasein was the vector through which ALL ontological Being comes into the world.)
I will now attempt to give an explanation of the constitution of Dasein.
The constitution of Dasein:
Descartes proposes that man can understand his being through the cogito. That is, ‘I think, therefore I am.’ Heidegger argues, in the tradition of Husserl, that the cogito is superficial because it makes no definition of the ‘AM’ in ‘I am.’ That is, Descartes’ maxim makes no account of ontological Being.
One of the most beautiful parts of this thought is a recognition that because Being precedes cognition, it is impossible to reach the root of our existence through thought alone. In order to think, one must have something with which to think. Dasein is always ahead of itself. One cannot think without already presupposing a mind, so to use thought as a scaffold of Being is dishonest.
Heidegger argues that we are FALLEN into being. That is, we awake already existing. We do not ask to exist before we find ourselves waking up; we simply awake already in the world. It is from this position that all philosophy should begin. I am biased here but for me this is a most brilliant and beautiful recognition.
It follows then, that the first and most essential aspect of Daseinic ontology (the ontological Being of Dasein) is that it is Being-in-the-world. What is the nature of Dasein’s being-in-the-world? It is more precisely understood, argues Heidegger, if we realize that Dasein exists ALONGSIDE the world into which he awakes. He interacts with the world through the process of what is called CARE.
Here is Donald the Dasein. Dasein is not really a subject but for our purposes we will depict him as a human (or my woeful attempt at drawing one). Dasein is really better depicted as a kind of cloud of Being existing around a human, extending into time.
Dasein has FALLEN away from the root of his being and is unable to reach it by thought.
In his falleness, Dasein exists alongside the world. He interacts with it through care.
Care is a relationship of interaction between Dasein and objects in the world. Heidegger gives a special meaning to interactions that reveal the ontological Being of objects. He terms them ‘disclosure.’ For example, Dasein discloses an aspect of human ontology when its Being is Being-towards-wanting-to-brush-its-hair. That is, the state of its ontological Being IS ‘towards-wanting-to-brush-its-hair.’ In doing so, Dasein also discloses an aspect of the ontological being of a comb – which we have outlined above. Through care (the need to brush hair) Dasein discloses the Being of a ‘comb’.
Dasein is that which instills the meaning of Being.
Now, care can form different categories depending on Daseinic need. ‘Handiness’ is one such category. The ‘equipmental’ ontology of a hammer is disclosed when Dasein is ‘Being-towards-wanting-to-drive-a-nail.’ The ‘hammer’ may be disclosed differently as a ‘weapon’ if Dasein is ‘Being-towards-wanting-to-smash-someone’s-face.’ Dasein discloses the ontological Being of ontic items and in so doing discloses its own ontological Being. The least significant category is ‘Being-towards-merely-contemplating’ things objectively present.
Care fashions matter into meaning. The same matter may alternately become a hammer, a weapon, a prop, an opener, a weight, a wedge etc. Dasein shepherds Being.
I find this idea fascinating. Everything you see is ontologically disclosed through care. That is, at your computer desk now you are ontologically disclosing the face of the monitor in your ‘Being-towards-wanting-to-understand.’ You are disclosing the metal object that perhaps sits adjacent as ‘merely-being-present,’ however, if you grow thirsty you will disclose your Being as ‘towards-wanting-to-drink’ and its Being will become ‘A can of coke carrying liquid to relieve thirst.’
So, Dasein is FALLEN into the world and its states of being derive from Being-in-the-world. The structure of its consciousness is of particular interest.
The construction of Daseinic Consciousness:
We spoke earlier of Dasein’s unreachable ontological root. That is, the root of its Being. It cannot be reached by thought. Dasein’s thought is always ahead of itself because it is ahead of its being.
Dasein attempts to gaze back at its primal Being, its root, but is unable to reach it. Fallen Dasein is a projection from this root. Dasein is THROWN into Being as a projection FROM this primal, unreachable foundation of Being. In what, for me, is the most breathtaking analysis of Being ever proposed, Heidegger notes that this phenomenon results in ANGST.
Angst arises in Dasein because it awakens to Being in a state of bewilderment. Angst is Dasein’s concern over its continuation. Dasein exists in time. Time is the horizon of its Being. Each and every behaviour Dasein exhibits is ultimately ‘Being-Towards-Death.’ THIS is angst; this concern over our projected THROWNNESS. Angst is a recognition of death; a recognition of Dasein’s own finitude.
Dasein’s Fallen-self attempts to desperately turn back into itself and search for its primal Being, trying to avoid the angst of Being-in-the-world. Such a process echoes in the cries of mankind – WHY AM I HERE? WHAT IS THE MEANING OF LIFE? It is amazingly profound to me that Heidegger ANSWERED these questions by realizing that they arise not as objectively meaningful queries (we have already discussed how all philosophy must begin with man already ‘in-the-world’

but as the defining tenet of human ontology in themselves. That is, simply, beautifully and magnificently: the question of ‘why am I here?’ is precisely, in itself, in its asking, the answer to ‘what is the meaning of human Being?’ These doubts, these questions fundamentally ARE human. They are what defines human consciousness. Once this is realized, the errors of depression and despair are seen as collateral occurrences to Dasein’s inability to comprehend its own ontological root.
I like to see Dasein’s unreachable primal self and Dasein’s Fallen self as two strands forming a kind of double-helix structure. The fallen self turns in on itself and attempt to reach the primal self - which it cannot - and is in turn thrown back out through projection into the world. The Helix continues on into time, continually projecting against itself and being thrown back into the world, ending abruptly at death.
Dasein also interacts with what Heidegger terms the ‘They-self.’ The they-self again is not a subject. It exists in consciousness as a protocol of ontological Being. The Being of They-self corresponds to what other people think. One must not Be ‘walking down the street naked’ because ‘They-self’ disagrees with it. One must not Be ‘rocking the boat’ because ‘They-self’ would not approve. One must Be ‘fearing the word of the media’ because it voices the collective ‘They-self.’
Heidegger proposes notions of authenticity and inauthenticity in Dasein’s Being. Inauthentic Being arises when Dasein wallows in its falleness so that its being exists solely in either They-self or in ‘merely-being-contemplated’ observations of ontic items. Inauthenticity is an attempt to escape a recognition of death (see the hordes of people buying cartoon dvd collections? They are, for Heidegger, ‘inauthentic’
Capability exists, however, for Dasein to become ‘resolute.’ Dasein must become accepting of its own death as a necessary part of its ontological beauty. Through ‘guilt,’ Dasein is called away from its submergence in inauthentic Being and rises to awareness of itself as shepherd of Being, disclosing it ontologically in time. Dasein is a vector through which meaning is instilled in the world. Dasein ek-sists. That is, it (ek) stands out from existence.
Heidegger defines ‘The Earth’ as a realm that Dasein has not disclosed the being of – it is slightly similar to the unknowable noumena (thing in itself). The World is the realm of Dasein. Dasein discloses things from Earth to World in a beautiful, gentle and wondrous array of varieties. Dasein’s gaze frames the Earth as World. Heidegger would later go on to argue that works of Art – particularly poetry, which sees things ontologically and metaphorically - can disclose Being in themselves.
The meaning of life is to disclose Being. The Heideggerian Superman is a warrior poet! So to answer your questions: Dasein is human ontology; angst a recognition of its ‘being-towards-death.’