Ok.. here is a short guide to greek food.
Mousakas: it consists of layers of potatoes, ground beef, aubergines and on top a layer of cream. all of this is cooked in the oven. it's very good indeed.
Feta: it's a type of white cheese. actually it should have this name only if it's produced in greece. goes well with a bit of olive oil and oregano.
come to think of it, everything goes well with a bit of olive oil and oregano.
Tzatziki: it's some kind of sauce that is made of yoghurt, cucumber and garlic. if you're going to kiss make sure you've both had of this.
Souvlaki: depending on the dialect you use, it can either mean small pieces of meat grilled on a stick, or a whole different thing.
If we're talking pieces of meat on a stick, it can come in various sizes depending on the restaurant (typically bigger in restaurant, smaller in take-away places). You can choose whether it's chicken or pork. The typical is pork, but i prefer chicken.
The whole different thing is: pita on the outside (you can compare this with the outer thing in tacos or tortillas, dunno), and inside the meat, tomato, fried potatoes, onion, tzatziki. Basically you decide what to put in. As for the meat you can choose to put inside, it can be chicken/pork pieces (without the stick) or gyros.
Gyros IS a huge chunk of meat (pork) that is on a metal stick and goes around slowly over the grill. when you ask it in your souvlaki they cut with a knife the outer parts that are cooked, put them in your pita, and the rest stays there to get grilled.
I suppose you can have this as a different dish too, but i don't really know cause it's not my thing.