Some hematomas form into welt-like formations that are hard to the touch. Such a
formation is a sac of blood that the body creates to keep internal bleeding to a
minimum. In most cases the sac of blood eventually dissolves, however, in some
cases they may continue to grow or show no change. If the sac of blood does not
disappear, then it may need to be surgically removed.
Hematoma is a bleeding into the skin, subcutaneous tissues (under the top layer of skin), or mucous
membranes ("skin" covering your mouth and lips). Purpura are flat areas where
blood has collected under the tissue. A hematoma is a larger collection that forms a lump.
Hematomas exist as bruises (ecchymosis), but can also develop in
organs. There are two kinds, an intermuscular and intramuscular hematoma
Here is a picture of a hematoma on an injured finger