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A hematoma is a larger collection of blood, usually caused by surgery, injury, or a greater trauma. Hematomas will usually reabsorb into the body, like a bruise. However, depending on the size, location and cause of the hematoma, the area may need to be drained surgically, or take a longer period of time to resolve.
Now, to answer your question, a subdural hematoma is basically bleeding into the space between the brain cover (dura mater) and the brain itself. More specifically, there are blood vessels running through the brain and in the spaces between the outside of the brain and the inside of the skull. During a brain injury, any of these vessels can tear and bleed.