Testosterone is a hormone that's important for the normal development of male sex organs and the maintenance of secondary male characteristics. It also has other effects, including muscle mass, bone density, fat distribution, red cell production, energy levels, and mood. Testosterone therapy may be appropriate if testosterone deficiency occurs naturally or as the result of certain diseases or medical treatments. Information can be found here.
The primary treatment goal for testosterone therapy is to improve testosterone levels, but it can also help with libido and erectile dysfunction. Testosterone therapies include testosterone injections, skin patches, gels, and other formulations applied to the body's surface (or taken by mouth). Talk to a doctor about how testosterone might affect your health before deciding on an option. See here for information about Testosterone Therapy, A New Treatment Option for Men.
The most common treatment options are injections, skin patches, gels that are applied to the skin surface without the need for needles (such as Androderm), and other formulations taken by mouth such as tablets you take orally like Viagra. Injections are taken once every two weeks or more often, while testosterone gels and tablets are typically taken daily.