Also known as spermatozoa or spermia. Male gamete or sex cell, posessing a motile flagellum for movement and a pointed head consisting mainly of nucleus. The base of the flagellum is lined with numerous mitochondria for powering movement; these organelles are discarded before the sperm penetrates the ovum. Enzymes present in the acrosome or 'nose' part of the sperm act on the outer parts of the ovum, altering it's properties so that a sperm may evenually penetrate - note that the enzymes from many hundreds of sperm are required for this task.

Note that, despite what Webster says, sperm is NOT semen (although the term sperm is sometimes used instead of semen - usually anong teens).
During spermatogenesis, sperm do not become motile until sometime during their journey down the tail of the epididymis. Once ejaculated, sperm are very active, and can move as fast as 20 microns a second. The flagellum makes whipping, undulatory movements, causing the sperm to rotate about it's long axis as it moves forward.

Sperm cels are the smallest in human body (roughly 175,000 sperm are needed to match the weight of one egg cell) and here are about 60,000 sperm per mm^3 of ejaculate. Sperm may survive outside the body for several days. They can survive in the fallopian tubes for a similar amount of time, and can be (and have been) extracted alive from the ejaculatory ducts some days after death.