The 12-bar blues is the cliché blues progression. Essentially, you have 12 bars and three chords.
From basics, the I chord is the root chord of your key, the IV chord is the fourth scale degree and the V chord is the fifth. In E, that's I = E, IV = A, and V = B. These are major chords, although in blues they are often played as dominant 7ths i.e. E7, A7, B7.
The 12 bars are divided into three musical phrases, each 4 bars long. This corresponds to three lines of lyrics (the lyrics themselves typically only cover 2 bars of the 4):
There's a red house over yonder, that's where my baby stays,
There's a red house over yonder baby, that's where my baby stays,
Ain't been home to see my baby in 99 and one half days.
Throughout the first line, you're playing the I chord. On the second line, you go up to the IV for two bars and then back to the I. In the third line, you play one bar on the V, one on the IV, and then back to the I for the rest.
The last bar of the whole thing may hop back up to the V chord. This is the "turnaround."
This is easier to hear than to explain. Well known 12-bar blues include Red House (Hendrix) and Crossroads (Cream/Robert Johnson).
For more detail (including downloadable backing tracks so you can hear how this sounds), check out Olav Torvund's guitar pages under blues guitar lessons:
http://www.torvund.net/guitar/BluesGuitar/index.asp
This is the absolute stereotype of the blues. The 12-bar is really just a starting point -- there are many more songs that don't fit into this well-defined box.