Work makes you mad. Simple - at least according to my high school chemistry teacher.
Actually, not so simple. Work is the product of a force (mass multiplied by acceleration) and a distance; in other words, a force does work when it moves an object some distance. So, when you lift a rock above your head, you have done work. Obviously.
The units are odd too. In the SI system, work is measured in Joules (Newton-Meters). To us Americans working on cars, it is measured in foot-pounds (feet for the distance and pounds for the force). Funny, eh?
You can read more at http://en.wikipedia.org/wiki/Work_%28physics%29.