Modern Progressive Rock had its conception in the early '70s with groups like Yes, Rush and ELP (Emerson, Lake and Palmer). These groups were already into Rock music in their early years but wanted something more than a three chord love song which most bands were recording at the time (and being fairly successful at it.) They wanted something that would take your ears on musical journeys through the vortex of time and space, to the tallest mountains, and to the lowest depths of the vast oceans.Many Prog Rock songs also had faint but sometimes strong political and environmental messages. For example the song "Anthem" by Rush was based upon the short story of the same name written by Ayn Rand, a prolific individualist writer, about a person in the future who learns about being an individual in a world where people are known by numbers instead of names.
The 2112 concept by Rush was also loosely based on Rand's books and is about a man that finds a guitar in a cave in a similar futuristic world where the only music is what comes from the priests and temple of Syrinx (rumored to be very boring music).
ELP (Emerson, Lake and Palmer) took a more lighthearted approach to their concept songs, writing a long concept about a half-armadillo / half-tank named Tarkus and how it progresses through its life and encounters and fights with other strange beasts like the Manticore, a large half-lion / half scorpion. Tarkus finally rolls its little tank body into the sunset and supposedly gives up the ghost in the desert.
Yes was another group that started out with fairly simple songs and progressed until they have become known as one of the most lyrically and musically complex Prog Rock bands in history. Even their album covers, all painted by artist Roger Dean, convey an almost mystical world, with large arching yet delicate rock formations and animals in strange environments. (See an example of R. Dean's work below).
Listening to the awesome concept songs like "Starship Trooper", "Close To The Edge", and "Siberian Khatru" most people enjoy the music so much they don't think about what the lyrics mean. When you do read the lyrics they are more like an abstract work of art or a set of thoughts and ideas. If you've ever pondered over a Salvador Dali´ or an Andy Warhol painting you will know what I mean.Prog Rock took to hibernation in the early and mid '80s but now I believe it is starting to wake up again in the '90s with new releases by Yes, Rush and Sting. Also some new groups taking up the Progressive style include Tori Amos, and Tool, a darker, heavier side of Prog Rock.The increasingly popular record labels Magna Carta and Metal Blade have brought progressive music out into the open recently with bands like Shadow Gallery (Magna Carta), Lemur Voice (Magna Carta), Kings X (Metal Blade), Fates Warning (Metal Blade), and some great compilations and tribute albums like Liquid Tension Experiment (Magna Carta), featuring John Petrucci (Dream Theater) on guitar, Mike Portnoy (Dream Theater) on drums, Tony Levin (Peter Gabriel, King Crimson) on bass, and Jordan Rudess (Dixie Dregs) on keys. (My personal favorite BTW |-). A few other great tribute albums released by Magna Carta are: Tales From Yesterday (A tribute to Yes), Working Man(A tribute to Rush). All feature a plethora of progressive artists and are definitely worth checking out if you like Yes, Rush, or any kind of Progressive music.
A personal favorite right now is a band called Dream Theater. They have created their own unique style of Progressive Metal in the style of Rush and have been rocking the progressive world lately. They have been putting out albums since the early '90s and are one of the most respected Progressive groups today. Members include:
Ozric TentaclesAnother group I am into right now is called Ozric Tentacles. They are a Progressive / Instrumental / Ambient / Techno / Guitar band if you would like to label them. It is very unique music, from pure ambient to jammin' pulsing guitar whining and anything in between. You must listen to appreciate...
Ocean Machine (Devin Townsend)
Ocean Machine: Biomech Vai: Sex and Religion
ELP (Emerson, Lake and Palmer) 
Stephen Caudel 
