What is progressive rock (prog rock)? According to Prog Archives:
“Progressive rock (often shortened to prog or prog rock) is a form of rock music that evolved in the late 1960s and early 1970s as part of a “mostly British attempt to elevate rock music to new levels of artistic credibility.” The term “art rock” is often used interchangeably with “progressive rock”, but while there are crossovers between the two genres, they are not identical.
Progressive rock bands pushed “rock’s technical and compositional boundaries” by going beyond the standard rock or popular verse-chorus-based song structures. Additionally, the arrangements often incorporated elements drawn from classical, jazz, and world music. Instrumentals were common, while songs with lyrics were sometimes conceptual, abstract, or based in fantasy. Progressive rock bands sometimes used “concept albums that made unified statements, usually telling an epic story or tackling a grand overarching theme.””
The most useful prog rock resource I have found is Prog Archives. This site has an active user community, an extensive list of prog bands, reviews, and hundreds of songs that are streamed for free.
Here are some more prog rock resources.