A useful punctuation mark used in Spanish. You use it when you begin asking a question. Unlike the question mark, which can only go at the end of the sentence, the inverted question mark goes at the beginning of the question phrase.
¿Comprendes? Es bien fácil, ¿no?
(The inverted exclamation mark works the same way.)
In HTML, it's ¿ and in Unicode it's U+00BF (191 decimal).