A spermatid is an intermediate stage in sperm growth in the human male. Each spermatid contains 23 chromosomes. That is, each one is the "male" half of a cell, or gamete, which contains half of the genetic material needed to create a human being. The other half of the genetic information will be supplied by the female egg. Before they are able to fertilize an egg, however, a spermatid will need to undergo a maturation process.
To understand fully what a spermatid is, it's necessary to know the basics of sperm production and where the spermatid stage fits into the whole. The name "spermatid" is very similar to the names given to many other stages of the cell development. Sperm production begins at puberty in the human testes—these sex organs hang free from the body in order to maintain the optimal temperature for sperm production. Inside the testes are seminiferous tubules, which are lined with Sertoli cells and contain spermatogonia, or "baby" sperm cells. Each of these cells contain 46 chromosomes, and through mitosis, the spermatogonia will then divide into sets of four spermatids.
The four spermatids each contain only half the deoxyribonucleic acid, or DNA, needed to make a human being—23 chromosomes. During the time of their maturation, they are connected by strands of cytoplasm. This period of sperm creation involves the spermatids incorporating into themselves random pieces of the parent DNA to create variations which ensure the generic information each sperm carries is unique.
The mitosis process which divides spermatogonia into spermatids is a long one, taking up to three weeks. The process is put into motion by several hormones, the most well-known of which is testosterone. At the end of their maturation process, spermatids will have changed into the familiar spermatozoa, which is what many people think of when they hear the word "sperm."
Though they have a head and a tail, the spermatozoa will not yet be fully mature. They will be moved to the epididymis tube, where they essentially learn to swim and gather strength to penetrate a female egg. After two weeks, they are finally stored in the vans deferens, where they'll be mixed with milky fluid from the prostate before ejaculation. The entire process of sperm development is called Spermatogenesi. In this process, spermatids are an important, if intermediate, part.