Although the genre is most often associated with seventies bands like Yes, Rush, Genesis, and Emerson Lake & Palmer, prog rock's origins actually date back to the sixties, where the burgeoning free-form FM rock stations of the time designated the new format as "progressive rock". In that respect, you can actually count bands like The Doors and Cream among the very first prog rock bands.
By the seventies, the term grew to be more associated with an ultra serious brand of rock, where the music invariably featured very long songs with intricate structures and time changes played on synthesizers as often as guitars. Prog rock musicians also soon became known for dabbling in other genres ranging from jazz to even baroque and classical.
Today, prog rock lives on with modern-day practitioners ranging from Porcupine Tree and Marillion to heavier prog bands like Tool. Prog fans are also among the most devoted you'll find anywhere, blogging about their favorite prog bands at sites like ProgRock.com, Proggnosis and ProgRock.org.