The cells in human beings have each one in its nucleus 23 pairs of chromosomes. Each progenitor provides their descendants with half of the genetics information, in the form of a chromosome of each pair. 22 of these pairs are called autosomes and the last one corresponds to the sexual chromosomes (X or Y).
Traditionally the pairs of chromosomes are described and named according to its size, from pair 1 to 22 (from large to small) plus the pair of sexual chromosomes previously mentioned. The chromosome 21 is actually the smallest one, so that it should occupy the place 22, but because of a mistake in the Convention of Denver in 1960, in which the Down syndrome was assigned the pair 21, and still exists nowadays, it was then for practical reasons left to this nomenclature.
The chromosome 21 contains approximately 1% of the genetics information of an individual in more than 400 genes, although nowadays it is only known accurately the function of a few of them.