Psychedelic rock is a genre of rock often (but not always) inspired or influenced by psychedelic (psychotropic) drugs, especially LSD and marijuana.
The musical style often incorporates flowing, hypnotic rhythms and use of guitar, vocal and organ effects, most notably the Wah-Wah pedal which is either used to make a fast, funky but psychedelic sound or used slowly to create a psychedelic wave of guitar sound, and Hammond organ used to make Blues psychedelic harmony.
There is a little debate as to which band was first known as being psychedelic rock but it is likely that it was either 13th Floor Elevators or The Charlatans (not the more recent UK band with the same name). Both of these bands had produced psychedelic rock albums around 1964/1965, 2 years before The Beatles released Revolver which included a very psychedelic song called Tomorrow Never Knows. Which ever band was the first, the Elevators seemed to be the epitome of the genre right from their first album The Psychedelic Sounds Of The 13 Floor Elevators which had psychedelic LSD inspired artwork and writings on the cover.
Much of The Jimi Hendrix Experience's output was psychedelic rock driven, as was Cream's.
In the UK at the end of the 1960s and into the 1970s bands such as Pink Floyd carried the psychedelic rock banner starting off with the first album Piper At The Gates Of Dawn and carrying on to albums such as Meddle and The Dark Side Of The Moon.
There is some blurring between the genres space rock and psychedelic rock especially with bands such as Hawkwind.
In the 1980s British bands such as Spacemen 3 and Loop were playing what could be described as psychedelic rock but with a more minimalist and drone based sound combined with kraut rock drumming and simple open stringed guitar riffs with lots of use of Wah.
Contemporary bands that could be described as psychedelic rock include The Warlocks, The Black Angels, KingBathMat& Dead Meadow