Question: What is a riff?
In songs, the lyrical phrase that is repeated and summarizes what the song is about is called a "hook." In terms of the music itself, the series of notes, chord pattern or musical phrase that are repeated is called a "riff."
The word riff is used in such genres as pop, rock and jazz. An example of a song that has a memorable riff is "Smoke on the Water" played by Ritchie Blackmore of Deep Purple. This song has a rock riff that is played using the G pentatonic scale (G, A, B, D, E). It is memorable yet simple to play, that's why it's so popular and most beginning electric guitar players learn to play it first. Watch Ritchie Blackmore as he shows how to play the "Smoke on the Water
When we speak of classical music however, we call the repeated musical phrase or pattern as ostinato. One of the most popular example of this is "Canon in D" by Pachelbel. "Canon in D" is one of the most recognizable pieces of classical music and uses the chord progression Dmaj-Amaj-Bmin-F#min-Gmaj-Dmaj-Gmaj-Amaj. Listen to Pachelbel's composition here.