What exactly is "12-bar blues"?

Its an old and well used three chord progression pattern, using the first, fourth, and fifth notes of a scale.

1-1-1-1 4-4-4-4 1-1-1-1 1-1-1-1
1-1-1-1 4-4-4-4 1-1-1-1 1-1-1-1
5-5-5-5 4-4-4-4 1-1-1-1 1-5-5-5....repeat.

4 bars per line (measure) x 3 = 12

This is just one variation of many, but it'll get you started.

http://en.wikipedia.org/wiki/12_bar_blue...

This should help.

Short and to the point!!!

The 12-bar blues has a distinctive form in both lyrics and chord structure which has been used in songs in many forms of popular music. Most commonly, lyrics are in three lines, with the first two lines almost the same with slight differences in phrasing and interjections:

I hate to see the evening sun go down,
Yes, I hate to see that evening sun go down
'Cause it makes me think I'm on my last go 'round

The chord progression is simple to identify after some study and attention as it rises and falls in a regular and very familiar pattern. The blues can be played in any key, but guitar and bass players prefer open chords, that is, chords with several open strings: E-A-B7 or A-D-E7. Keyboardists may prefer C-F-G7 or G-C-D7.