its called a boil, wen the pus get on other areas of the skin it can cause more boils to pop up. there highly contagious so dont share anything with anyone else. I would go to the ER immediately because it is an open wound staph(Staphylococcus) infection and get it treated asap!!!
Find a dermatologist in your area to get that checked out. It could be as simple as acne, but I wouldn't risk it.