There are several kinds of brain injury, including blows to the head,
bleeding, and direct damage to the brain itelf, such as from a
gunshot wound.

In the case of a blow to the head (such as might occur if you fall
or are in a car accident), the force of the blow causes the brain to
be forced against the side of the immobile skull and to swell. In
addition, there may be bleeding into or around the brain. The brain
swells when it is injured, just like any other part of your body.
The difference is that your brain is encased in the unyielding bony
skull, where there is very little room to accomodate the swelling.
Once the limited extra room in your skull is used up, the brain
tissue starts to push down through the opening at the bottom of
the skull where the spinal cord continues down the vertebral
column.

In the case of an aneurysm or stroke, a blood vessel in your head
bursts or a clot stops the flow of blood and oxygen to the brain
tissue. Once again, the injured brain itself begins to swell and more
brain injury occurs because the skull cannot accommodate the
extra fluid.

Finally, if there is a gunshot wound (or similar wound involving a
foreign object penetrating the skull), the brain is directly damaged
by the bullet's tearing through the brain tissue, possibly ricocheting
within the skull and causing bleeding, swelling, and further damage
that way.