Acne, also known as Acne vulgaris, is a skin disease that occurs when pores or follicles are clogged with dead skin cells and sebum. Bacteria growth escalated within the clogged pore/follicle resulting in infection. Our body natural response to remove the harmful stimuli (the escalated bacteria growth) is inflammation. You can tell when the skin is inflamed when your skin looks red and bumps starts to develop, some bumps might contains pus, which if treated or handled improperly may cause scarring (acne scars).
Acne primarily appears in areas of the skin with relatively higher concentration of oil glands, this includes face, upper chest, and back.
Also see: How to treat acne