A hematoma is a more solid, three-dimensional collection of blood that has escaped the blood vessels, while a bruise is a flatter collection of escaped blood, according to MedicineNet. Other names for a bruise are contusion and ecchymosis, which is a very large bruise, reports MedlinePlus.
A bruise is found right beneath the skin, according to MedicineNet, while a hematoma is found under the skin or in an organ. A hematoma can be tiny or very large and dangerous. This danger is especially pronounced if the hematoma is in the brain, where it has no place to go if it enlarges.