Cell division has three purposes for an organism: reproduction, growth and maintenance. For single-celled organisms, this is their direct and only method of reproduction, and it serves no other purpose. For multicellular organisms, cell division is a step in reproduction and is necessary for growth and maintenance.
Cell division occurs in two different basic ways. The first, and by far the most common, is mitosis. Mitosis is the only way in which single-celled organisms divide, and the vast majority of cell division for multicellular life is mitosis as well. In mitosis, a cell divides into two smaller cells, known as daughter cells, with identical genes. In single-celled organisms, these daughter cells generally have the same basic characteristics as the parent cell. In multicellular organisms, these can either be similar to the parent cell or, as in the case of the production of blood cells, very different, although the genes are the same.
Meiosis, on the other hand, produces gametes, daughter cells that are structurally different from their parent cells and contain only half the number of genes. Theses gametes are used for sexual reproduction in multicellular life and must combine with another corresponding gamete before they can begin growing via mitosis.