A hematoma occurs when blood vessels are broken and an opening in the skin large enough for the blood to escape is not present. Since the blood cannot leave the body, the blood is still released from the vessel, but it bleeds into the surrounding tissues. This typically leaves a dark marking around the area where the broken vessels occurred, as well as some swelling. The area around a hematoma is extremely sensitive.
It is possible to confuse a bruise with a hematoma, but they are not the same. A bruise is a much smaller vessel breakage and there is generally no swelling. Most bruises disappear fairly quickly and result from an impact against a part of the body. A hematoma is generally achieved from an infiltration of the body, commonly from a needle which was moved around close to a vein or artery and a blood vessel was broken.
There is no specific "cure" for a hematoma. It may take several days for the hematoma to drain, the swelling to go down, and for the area to return to normal.