Pus is the yucky yellowish stuff that oozes out of an infected cut or pimple. Medically speaking, pus is made up of various types of white blood cells that have died.
Bacteria can cause infections, and when the infected area becomes inflamed, pus often forms and collects under the skin. A boil or pimple full of pus can be painful, and seeing pus in a wound is a sign that you should go to the doctor, who will prescribe antibiotics to treat the infection and may also drain the pus. The Latin pus means "matter from a sore" but also "bitterness or malice."