Blues
The lyrics (words) are usually about life, more specifically, the bad things in life. They are simple blues songs that are also based around the form of 3 phrases. The first phrase is set over the first 4 bars of music. The second phrase words are a repetition of the first phrase, this always happens and emphasises the message. The third phrase completes the "story" and rhymes (ends with the same sound) as the first line. This verse from  Robert Johnson's "Crossroad Blues" shows how these phrases work in a song: