12 Bar Blues is a pattern of chords which repeats every 12 bars. There are hundreds of well known songs based on this chord progression, i.e., they contain basically the same chords in the same order. 12 Bar Blues is commonly used in Blues, Jazz, Funk, Soul, Rock, Country and Folk music. Most 12 Bar Blues contain lots of seventh chords.
The following example is a 12 Bar Blues riff in the key of G. A riff is a repeating pattern of notes which may be altered to fit a series of chord changes. When a song is said to be in the key of G, the most important chord (and usually the first chord) is some kind of G chord (usually G or G7). This pattern of chords will probably sound familiar to you. Remember to use alternate picking. End this 12 Bar Blues by playing a G note.