A tort is a civil wrong for which a rememedy may be obtained. Basically, a tort is something someone else did wrong that caused you injury and for which you can sue. In this context, driving negligently, hitting someone and causing injury is a tort, and the injured driver can sue the one who caused the accident.
A tort can be intentional -- like an angry punch in the nose -- but is far more likely to result from carelessness (called "negligence"), such as riding your bicycle on the sidewalk and colliding with a pedestrian. While the injury that forms the basis of a tort is usually physical, this is not a requirement -- libel slander and the "intentional infliction of mental distress" are on a good-sized list of torts not based on a physical injury.