I guess you could call them 'melodic motifs'. Riffs are basically just melodies, usaully played on guitar or bass, that are memorable and drive a song.
Relatively short patterns of notes, often repeated in a song -- sometimes a pattern characteric of a genre.
Some famous riffs:
Bass part in Peter Gun Theme
Note patterns in verses of Sweet Home Alabama
Repeated guitar line in Rock Lobster
Intro to Secret Agent Man
Intro guitar part to LaGrange
Single note verse guitar part for Black Dog, and of course most of Pagey's repeated guitar licks for many Zep songs.
Repeated single line guitar part in Nirvana's Come as You Are
The memorable little guitar lick at the intro and end of every chorus in Harrison's Something
The simple minor to major third hammer-on used in many country tunes
-=tension & release=-
Have a look at some of the lessons on this site. For example, Easy Riffs of the 90's :)
From little things big things grow - Paul Kelly
Rhythmic motifs - short phrases played repeatedly
"Whole Lotta Love" is probably the best known as it's used throughout the whole song.
"Be good at what you can do" - Fingerbanger"
I have always felt that it is better to do what is beautiful than what is 'right'" - Eliot Fisk
Wedding music and guitar lessons in Essex. Listen at: http://www.rollmopmusic.co.uk
Let's not forget the riff in "Day Tripper" and the riff in "Satisfaction"!
Well we all shine on--like the moon and the stars and the sun.
-- John Lennon
uhh. Also... Eric Clapton's *dodges tomato* Layla *runs off stage*