Mary Sue: An original character that is introduced to the fandom and tends to be perfect. Some common characteristics are:

Eyes--usually aqua, gold, silver, or purple. Glowing.
Hair--Really long. Of an unusual color. Red is common, but so is blonde, gold, silver, and chestnut. Very silky and manageable. Often glows.
Skin--Very clear. Usually pale, for some reason. Glowing.
Name and Titles--Usually lengthy, containing "ae" combinations, several l's, a canon character's name put through a blender, or all three. Many titles, cheifly that of Princess, Lady, or the last name of a canon character are tacked onto this name.
Origin--Is usually a long-lost relative of a canon character. The most common ones are Harry Potter twins and Sirius Black daughters. Often, they're also tranfers from American schools, and/or self-inserts. In LOTR fic, they manifest themselves as "falls into Middle Earth" stories.
Abilities--The Mary Sue often has special abilities. These can be, but are not limited to, wandless magical spells, a different language (this is especially common with Fangirl Elvish), and healing powers. They are also usually incredibly sarcastic, or incredibly kind. Regardless, they are universally liked, and lusted after by their male counterparts. They are also good at everything. Can often communicate with animals, especially snakes and wolves.
Posessions--Typically own some kind of powerful necklace, sword, horse, or other animal.
Other notes--Mary Sues show themselves most often in Harry Potter, where many inexperienced authorettes are trying to insert themselves into their fandom. In LOTR, they come almost exclusively as elves, and Legolas falls for them.

Draco and Legolas are the two most abused characters, though Harry follows a close second. Sometimes the HP Trio is put down, the authorette thinking that she is being original.

People who write Mary Sues should expect to be mocked for their lack of originality. They are sullying the fandom, and giving the rest of us a bad name and a lot of crap to read through to find good fic. That is why we mock them. After many tries to beta read for Mary Sue writers, the community has come to the conclusion that such efforts are a waste of time. When they are beta-ed, the beta reader is attacked with whiny "it wasn't that bad" comments. Thankfully, I have recieved none of those. Yet.