From a thread in the WoW Raid & Dungeon forums.
Raiding is like playing in an orchestra. 5-man is like improvising jazz.
Amelia (60 Priest):
I think you badly overstate the skill required to do either of these things...more accurately:
"Raiding is like playing in a middle school marching band, 5-man is like open mike night at your local karaoke bar."