Hi Squirrel-monkey,
I think Blacksword has given you an excellent answer there, but he left it in the early 1970's, so if you don't mind, please indulge me while I add another chapter...
I think that the introduction of neo-prog music in the UK in the early 80's, mainly by Marillion, but also Pallas, IQ, Twelfth Night and more latterly Pendragon, was probably a rebellion against the music of its day.
The charts in 1983 was swamped by by 'New Wave' pop such as Duran Duran, Spandau, OMD & Human League, which had grown out of the toned down post punk movement. These bands were the direct result of Thatcherite Britain, where income tax had been reduced benefitting middle income Britain. People were encouraged to own their own homes, had more spending money than before, and we became quite a selfish, materialistic & superficial nation. The unions were broken, manufacturing plants were closed and there was a rise in white collar work. This was reflected in the nation's musical tastes, where 'New Wave' revolved around clean cut images, sharp clothes, with short and very catchy commercial three minute songs (I don't think we've ever really recovered!).
The neo-prog movement was a half way house between prog and pop. It was fashionable in the early 80's to pour scorn on bands like Yes and Genesis who worked tirelessly to reinvent themselves, so a return to fully blown prog would have been unsuccessful, and yet there was a desire to create 'rock' music with considerably more complexity and creativity than either the new wave movement in the UK or the AOR movement in the US... hence Marillion etc.
It is interesting to note that of the bands mentioned in the first line, only Marillion were commercially successful, as they only seemed to be given artistic freedom by record companies who were in turn becoming much more image conscious and maintained more and more control over their artists who they wished to 'manufacture'... consequently, Pallas had contractual problems with EMI and IQ with Virgin, both of whom had to compromise their music considerably in order to be produced.
I appreciate I've rambled on... but I hope that is useful for your assignment.
Cheers