What's a riff?

 A Riff is a short, memorable musical phrase. We often listen to the riffs at the beginning of the song and are repeated throughout it.

Smells Like Teen Spirit, by Nirvana

Satisfaction, by The Rolling Stones

Smoke On The Water, by Deep Purple

Sweet Child On Mine, by Guns N' Roses

Day Tripper, by The Beatles

Layla,by Eric Clapton

What's a Hook?

 A Hook is a repeated part of a song (lyrics and music) that is particularly pleasant and easy to remember. Examples of hooks: 

Lollipop by The Chordettes

YMCA, by Village People

MMMbop, by Hanson

Hey Ya!, by Outkast