The bodies of most animals and plants contain body (somatic) cells, eg skin cells, nerve cells, liver cells etc and gametes (sex cells). The body cells of most animals and plants each contain two sets of chromosomes in their nuclei.
Every chromosome in one set has a matching partner in the other set that has the same length of DNA and the same genes. Cells that contain two sets of chromosomes are said to be diploid.
In sexual reproduction new organisms are produced from the fusing of the nucleus of a male sex cell with the nucleus of a female sex cell. This is called fertilisation. Sex cells are also known as gametes. Male gametes are made by male reproductive structures. Female gametes are made by female reproductive structures.
A gamete contains a single set of chromosomes in its nucleus and is said to be haploid. Compare the diagram of a haploid fruit fly gamete to the diagram of a diploid fruit fly body cell.