An unintentional injury is an injury inflicted upon a person without the suspect's purpose of a motive whereas intentional is the opposite and the suspect has a motive behind the injury.
Intentional: Mark decides to get revenge on Matt for flirting with his girlfriend. He picks up the golf putter at their nightly outting of Go-Go Golf and smacks the metal end down on Matt's collarbone. The force cracks his collarbone.
Unintentional: Maria is driving but is slightly under the influence. She sees a man crossing the road in the middle of the night and attempts to swerve out of the way. She clips his calves and heels, causing burns/lacerations on his legs, but he is otherwise okay.