A pimple is a small pustule or papule. Pimples develop when sebaceous glands, or oil glands, become clogged and infected, leading to swollen, red lesions filled with pus. Also known as spots or zits, pimples are a part of acne. They are most likely to occur around puberty, but they can happen at any age. Areas of the skin that have the largest number of functional oil glands, such as the face, neck, chest, back and shoulders, are often affected. Symptoms include small tender bumps on the skin, often filled with pus. Sometimes, dead skin cells are not shed. They remain in the pores and get stuck together by the sticky sebum, causing a blockage in the pore.