A gamete is a haploid sex cell; i.e., it contains only one set of chromosomes in a diploid individual. During fertilization, male and female gametes unite to form a diploid zygote (the initial developmental stage of the embryo).
Gametes are produced by a special type of cell division called meiosis. During meiosis, each gamete cell obtains its own set of chromosomes (assorted independently from the organism's homologs), and these chromosomes undergo genetic recombination. Because of this, every gamete is essentially unique, a fact that helps to guarantee the genetic dissimilarity of siblings and fosters a huge amount of diversity within a species.