Testosterone is part of the androgen group of hormones and is a chemical messenger that is mainly produced in the Leydig cells of the testes in men and ovaries in women, although small amounts are also produced by the adrenal glands.
Like all hormones, it works by binding to a specific receptor to create a specific effect. Aside from its role in development of the sexual organs and reproductive function, testosterone also helps build and maintain muscle mass and bone density, increases red blood cells (which carry oxygen to all the tissues) and contributes to a sense of wellbeing.