Unlike normal, diploid cells, gametes contain only a single set of chromosomes. They are therefore called haploid. They are formed by meiosis, a specialized type of cell division, which reduces the number of chromosomes by half.
The only haploid cells in humans are sperm and egg. During reproduction, as the sperm fertilizes the egg, they fuse to form a so-called diploid zygote, the precursor of the embryo.