Relatively little is known about Shakespeare the man, which has fueled suspicion that his works were in fact written by somebody else and that he was merely an actor who served as a pen name. Many have put the case for his works to have been written by Francis Bacon or Christopher Marlowe (who would have had to fake his own death), but none of the collected evidence is even close to convincing.
Shakespeare is of course best known for his plays, many of which were adaptations of older stories, but he appears to have believed he was more likely to be remembered for his sonnet collection.
He is responsible for bringing many cliches into the English language, such as "It is a tradition more honoured in the breach than the observance" and "The better part of valour is discretion".
The Wikipedia article on Shakespeare - http://www.wikipedia.org/wiki/William_Shakespeare