An abscess is a localized collection of pus in any part of the body, caused by an infection. Here this poodle has a head injury that abscessed and swelled in a matter of a few hours.
"Abscesses occur when an area of tissue becomes infected and the body is able to "wall off" the infection to keep it from spreading. White blood cells, the body's defense against some types of infection, migrate through the walls of the blood vessels into the area of the infection and collect within the damaged tissue. During this process "pus" forms, which is an accumulation of fluid, living and dead white blood cells, dead tissue, and bacteria or other foreign invaders or materials."
By morning his abscess had ruptured on it's own and the swelling was down. He went for surgery, they drained it, took a sample and made sure there were no foreign bodies. They put him on antibiotics and he now looks, not to mention feels, much better!