A riff is generally accepted to mean an important passage in a song, or a sequence of notes which together are recognizable.

E.g.
-------------------------------------------------
-------------------------------------------------
-------------------------------------------------
---------5----------------------5----------------
------7--------------7-------7--------------7----
---0--------6--5----------0--------6--5----------

for "Enter Sandman".

Sometimes a riff is also called a "lick".

Hope that helps.