This is blues in Am, but the line is easy to transpose to any minor key. The rhythm is from songs like Help Me and Green Onions. It's just a shuffle, with a continuously repeating pattern of (1/4 note), (dotted 1/8 note, 1/16 note). The chord progression is all minor chords (i7, iv7 v7). The harmony is r,(r,r),b3,(4,b3) in each chord around the chord progression.

Note how the change in rhythm and the choice of notes in 12th measure mark the end of the verse and lead strongly into the start of next verse.

The next page has one variation of the riff ... playing some of the notes below the root ... along with a RealAudio examle of the riff.