What is 12-bar blues?

The 12-bar blues is a repeating 12-measure chord progression. In its plain form it spends four bars on the I chord, two on the IV, two back on the I, one on the V, one on the IV, and the last two turning back home (I, then V again to set up the loop). Verses are usually built as an 'AAB' lyric — a line sung, repeated, then answered — which fits neatly across the twelve bars. It became the harmonic skeleton of blues, rhythm and blues, and early rock and roll, which is why so many songs feel familiar the first time you hear them.