Riffs is what makes a song 'catchy' so that people would get drawn into the music so that they would like it. Riff's normally go  from 2 bars to maybe 16 bars. Signature music is the longed version of the word Riff. They can be easily made by just throwing in a bunch of notes from a scale. Like the one I made (to the left). You can add beats and maybe a backing. Rock bands are normally the era of music which contains the most riffs so that the audience doesn't get bored with just the same music. The riff allows people to recognize a piece of music because the riff gets stuck inside their head for a short duration of time.