The Rock music genre has many sub-genres, some very popular, some not so popular, but without doubt, the Progressive Rock is a complex and delightful sub-genre.
Developed in the late 60's and the early 70's, Prog Rock (another way to call the Progressive Rock sub-genre) pushed "rock's technical and compositional boundaries" and "...explored extended musical structures which involved intricate instrumental patterns and textures and often esoteric subject matter."
But not only that, the Prog Rock also includes elements from jazz, world music, even classical music! Instrumental are more common, but it also has lyrics, which often talk about abstract, fantastical, conceptual or strange things.
The concept albums are also ordinary in Prog Rock, which regularly talk about epic stories, like Rush's 2112. They are, frequently, long and may seem boring for many people, but a real Prog Rock fan will love them.
The precursors or Prog Rock may be Bob Dylan, The Mother's Invention and The Beatles, with poetry, Freak Out! and Sgt. Pepper's Lonely Hearts Club Band, respectively, showing a great art with their rock, bur earlier Beatles's albums also incorporated elements like unusual instruments.
Progressive Rock peaked it's popularity in the mid 70's, when the Prog Rock band's albums topped the charts, and the bands themselves did too. Many Prog. Rock bans were formed in this period, and some, like Kansas, became one of the most successful bands of Progressive Rock.
After it's peak, in the years 1976-1977 Progressive Rock started to decay; listeners no longer had patient for 30 minutes suites inspired by J.R.R. Tolkien and preferred Punk music
In early 80's Prog Rock saw a revival, with bands influenced by 70's bands but with new elements. The bands belonging to this period are often called Neo-Progressive or Neo-Prog.
Many Neo Progressive bands made their music simplified and making it more commercial.
Progressive Rock was an inspiration for Progressive Metal and for bands like Dream Theater or the more extreme Opeth.