Razor bumps (pseudofolliculitis barbae) develop after shaving when strands of hair curl back on themselves, grow into the skin, cause irritation, and make pimples. Razor bumps are sometimes called razor burn.
Razor bumps develop most often on the neck, but they can occur in any area that is shaved (such as the cheeks, underarms, pubic area, and legs and arms).
Razor bumps occur most often in people who have tightly curled or spiral hair strands. Using different shaving techniques can help relieve this problem.
WebMD Medical Reference from Healthwise