An abscess is a collection of liquefied tissue that's been enclosed somewhere in the body. This liquefied tissue is yellow, and commonly known as pus. It's created as a part of the body's natural defenses against infection, and an abscess can form anywhere there is a foreign invader in the body. These can be due to germs or bacteria, which are called septic abscesses, or due to drugs that are never absorbed into the body which are called sterile abscesses. Sterile abscesses usually scar over, whereas septic ones cause inflammation and generate pus.
When a germ is detected in the body, it activates the immune system and the white blood cells. The white blood cells gather at the infection site and begin to produce enzymes that will attack the germ like acid and begin to digest it. The germ may also produce similar enzymes, and a biological shoot out can commence. The collateral damage is that the germ, in addition to portions of surrounding tissue, will be liquefied by the enzymes and turned into pus. This pus is then collected and held in the area of the infection until it's taken away bit by bit to be disposed of by the circulatory system.
This activity often happens near the surface of the skin, and even on the scalp. The infection begins with inflammation and irritation, then pus begins to form and spread along the skin. An abscess will grow along the path of least resistance, which is why it often spreads along the skin. Sometimes, if the pus isn't disposed of quickly enough by the circulatory system, an abscess will burst. The pressure from the built up pus ruptures the abscess, and the pus pours out. This can often be beneficial in that the digested enzymes, germs, and tissue are removed from the body, but a ruptured abscess must be carefully cleaned and disinfected if this happens.