I've also heard that the characters of that show represent the seven deadly sins.  It's not a perfect theory, but it's kind of fun. 

I'll add what I remember:

Squidward: wrath

Mr. Crabs: greed

Plankton: envy

Sandy: Pride

Patrick: sloth

Spongebob: lust (don't remember why)

Gary: gluttony (running joke about him needing to be fed)