A bruise and a contusion are one in the same, they represent damage to, or a rupture of capillaries around the site of injury. A hematoma is a collection of blood (usually clotted) in a tissue or organ that comes from a damaged vessel.
Per Wikipedia: "Generally speaking, dermatologists prefer to differentiate between purpurae, petechia, and ecchymoses as descriptive technical terms. Other specialties such as internal medicine will frequently call all of these ecchymoses, because the finer distinction is not relevant in the particular case".
Some folks, myself included, refer to a superficial reddened or brownish bruise (such as a "hickey") as an ecchymosis, and a contusion is thought of as something deeper and goes through a few color changes before resolution.