An Event is anything significant that happens or is contemplated as happening. An event happens completely or not at all and is significant because it may affect some action. It is contemplated as happening as it could be a fact becoming true or could be a transition of an entity in the real world. It may be part of a business process...
via ibm.com
This is a good whitepaper on the basics, but I couldn't help hearing Mr. Miyagi's ghostly voice in the background saying "Wax on! Wax off!".