From Wikipedia:
Blues is the name given to both a musical form and a music genre[1] that originated in African-American communities of primarily the Deep South of the United States at the end of the 19th century from spirituals, work songs, field hollers, shouts and chants, and rhymed simple narrativeballads.[2] The blues form, ubiquitous in jazz, rhythm and blues, and rock and roll is characterized by specific chord progressions, of which the twelve-bar blues chord progression is the most common. The blue notes that, for expressive purposes are sung or played flattened or gradually bent (minor 3rd to major 3rd) in relation to the pitch of the major scale, are also an important part of the sound.
The blues genre is based on the blues form but possesses other characteristics such as specific lyrics, bass lines and instruments.
I think we are more after the genre rather then the strict form, but would like to better understand the form as well.
The 12 bar chord progression works as follows:
I - I - I - I
IV - IV - I - I
V - V - I - I
But there are loads of variations as well.