From allmusic.com (which I find to generally be a very good source): Progressive rock and art rock are two almost interchangeable terms describing a mostly British attempt to elevate rock music to new levels of artistic credibility. The differences between prog-rock and art rock are often slight in practice, but do exist. Prog-rock tends to be more traditionally melodic (even when multi-sectioned compositions replace normal song structures), more literary (poetry or sci-fi/fantasy novels), and more oriented toward classically trained instrumental technique (with the exception of Pink Floyd). Art rock is more likely to have experimental or avant-garde influences, placing novel sonic texture above prog-rock's symphonic ambitions. Both styles are intrinsically album-based, taking advantage of the format's capacity for longer, more complex compositions and extended instrumental explorations. In fact, many prog bands were fond of crafting concept albums that made unified statements, usually telling an epic story or tackling a grand overarching theme. In addition to pushing rock's technical and compositional boundaries, prog-rock was also arguably the first arena where synthesizers and electronic textures became indispensable parts of a rock ensemble. The earliest rumblings of progressive and art rock could be heard in the poetry of Bob Dylan and conceptually unified albums like the Mothers of Invention's Freak Out! and the Beatles' Sgt. Pepper's Lonely Hearts Club Band, all of which suggested that rock was more than just teenagers' music and should be taken seriously as an art form. Prog-rock began to emerge out of the British psychedelic scene in 1967, specifically a strain of classical/symphonic rock led by the Nice, Procol Harum, and the Moody Blues (Days of Future Passed). King Crimson's 1969 debut In the Court of the Crimson King firmly established the concept of progressive rock, and a quirky, eclectic scene was taking shape in Canterbury, led by the jazzy psychedelia of the Soft Machine. Prog-rock became a commercial force in the early '70s, with Emerson, Lake & Palmer, Yes, Jethro Tull, Genesis, and Pink Floyd leading the way. Meanwhile, a more avant-garde scene (dubbed Kraut-rock) was developing in Germany, and eccentric, unclassifiable bands continued to emerge in the U.K. By the mid-'70s, a backlash was beginning to set in; prog-rock sometimes mistook bombast for majesty, and its far-reaching ambition and concern with artistic legitimacy could make for overblown, pretentious music. Its heyday soon came to an end with the advent of punk, which explicitly repudiated prog's excesses and aimed to return rock & roll to its immediate, visceral roots. Still, prog-rock didn't completely go away. A number of AOR bands used prog ideas in more concise songs; plus, Pink Floyd, Yes, and Genesis all had number one singles in the '80s by retooling their approaches. A small cult of neo-prog bands catered to faithful audiences who still liked grandiose concepts and flashy technique; the first was Marillion, and many more popped up in the late '80s and early '90s.
To me, prog rock defines a style nowadays more than anything. That style, too, is pretty much dead these days. You had tons of bands in the late 60s and throughout the 70s going with a similar style of rock... i.e. straying from conventional song structure and using many instruments and flaunting great musicianship (King Crimson, Genesis [Peter Gabriel era], Tull, Banco del Mutuo Soccorso, Pink Floyd, Aorta, Atomic Rooster, Camel [first four albums], Buffalo, Czar, Burnin Red Ivanhoe, Hawkwind, the list goes on and on and on and on of bands mostly from Britain and the U.S., some from Italy and South America and other miscellaneous areas). The only modern band I've heard that resembles that true prog rock sound/style is Anglagard (leaving out the Swedish characters). 98 percent of what people call prog rock these days is neo-prog, as also is defined at allmusic.com. Prog rock akin to all those bands I mentioned truly is dead.
As far as bands that are progressive, I think Devil Doll really has a unique style. Also, Esoteric is a band I would consider progressive. But, like I said, I don't necessarily equate progressive with prog rock.. to me, it's more of a style and sound... It was very progressive back when it originated, but now, like Dag said, it's not... because it's been done by hundreds of bands in the span of 1965-1979!
