Sperm are tiny cells produced in the testes during sexual arousal, and they're designed to fertilize eggs. They have a head, which contains genetic material, and a tail that helps them move quickly through the female reproductive tract toward the egg. Sperm can live for several days inside the female body, increasing the chances of fertilization if intercourse occurs near ovulation.
Under a microscope, sperm appear as small, oval or tadpole-shaped cells with a dark center (the head)