A pimple is a result of a blockage of the skin's pore. It can be a pustule or papule.

Causes

Inside the pore are sebaceous glands which produce sebum. When the outer layers of skin shed (as they do continuously), the dead skin cells left behind may become 'glued' together by the sebum. This causes a blockage in the pore, especially when the skin becomes thicker at puberty.The sebaceous glands produce more sebum which builds up behind the blockage, and this sebum harbours various bacteria including the species Propionibacterium acnes.

Over-The-Counter Medications

Common over-the-counter medications for pimples are benzoyl peroxide and/or salicylic acid. Both medications can be found in many creams and gels used to treat acne through topical application. Both medications help skin slough off more easily, which helps to remove bacteria faster. A regimen of keeping the affected skin area clean plus the regular application of these topical medications is usually enough to keep acne under control, if not at bay altogether. 1-2% of the population is allergic to benzoyl peroxide treatments.

Prescription medication

Severe acne usually indicates the necessity of prescription medication to treat pimples. Prescription medications used to treat acne include isotretinoin, which is a retinoid. Historically, antibiotics such as tetracyclines and erythromycin were prescribed. While they were more effective than topical applications of benzoyl peroxide, the bacteria eventually grew resistant to the antibiotics and the treatments became less and less effective. Also, antibiotics had more side effects than topical applications, such as stomach cramps and severe discoloration of teeth.

Popping

The popping of pimples is never medically recommended. It can lead to bleeding and scarring. But it is a popular method to relieve the discomfort some pimples inflict, it also immediately destroys the pimple. Popping is done by applying pressure around the area, forcing or "popping" the clog out of the pore. The result can be varied, anything from nothing but a small red mark that will heal in less than a day, to a small puncture wound that will scab over and heal, or a scar. Bleeding can be none to little, or it could bleed heavily.

from : http://en.wikipedia.org/wiki/Pimple