What the ham sandwich is a Mary Sue?

Also, a "Mary Sue", or if a male, a "Gary Stu", is a character with no real character flaws.  The overly good, invincible hero.  Think Drizzt, Aragorn, Will Turner, Wesley Crusher, etc.

A Mary Sue / Gary Stu can also be a character who exhibits a lot of common cliches, such as the "Angsty Sue" (Rand Al'Thorne), "Rebel Sue" (Snake Pliskin), "Anti-Sue" (villains with no redeemable qualities), "Outcast-Sue" (Wulfgar), or "Self-Sue", as someone said above, when someone basically inserts themselves.