What are riffs?

Riffs are notes repeated again and again, usually in Rock Music or Rap "Music".

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Example.

Smoke on the Water is an Iconic and Famous Riff. Play it and you will recognise it instantly.
E G A, E G B A, E G A, G E etc

Thank you!!

Actually, the term was appropriated by rock musicians and rock journalists from jazz. The term was very commonly in use during the swing era (1930's). The classical term for a repeating melodic passage is ostinato (which literally means obstinate or stubborn.)