This is a bit old, but damn if I'm not a sucker for internet memes.
LAist does a decent job of explaining the phenomenon to those of you fortunate enough not to have been initiated yet:
If you don't know what a Rick Roll is and you've never experienced a two-day long orgasm or won a free iPod, welcome back to the internet. We've missed you the past year. There's even a Rickroll database out there for your protection.