A typo, short for typographical error, is a misspelling produced by careless typing. Leaving more than a few of these in your fic is a sure sign that you didn't engage the services of a competent beta-reader.
Some typos result in gibberish, which will usually be caught by a spell-check program. Others, however, result in real words that mean something the author did not intend. This kind of typo can wreak all sorts of havoc in a Word World, ranging from unintentional comedy to unintentional horror. Typos of canon names generate different minis depending on which continuum the name is from.
In OFUM and on the PPC Board, typos may be blamed on Toey, the One Toe Ring of Causing Typos. Damn you, Toey!