In the United States alone people spend 3.5 billion dollars a year on hair loss treatments. Since this is obviously a very lucrative market it is a crowded one, with a range of products claiming to treat hair loss. Hair regrowth products range from transplant services like those of Bosley to FDA approved drugs to hair thickening shampoos and conditioners. Here are some of the most well-known hair loss treatments and how they claim to work.
Perhaps the most commonly used hair loss treatment in America is minoxidil, which is sold under the trade name Rogaine and under various store brands. Minoxidil is a topical medication that is applied to the scalp daily to stop hair loss and stimulate the growth of new hair. The new hairs are typically fine and somewhat small, but the many people find the results satisfactory. 
Propecia is a drug that was first used to treat benign prostate enlargement. It blocks the conversion of testosterone to DHT, which is the leading cause of hair loss in men and women. Propecia can only be used in women who will not become pregnant, because it causes birth defects. In a small percentage of men, side effects such as impotence and mood disorders cause them to stop taking the drug. It does not work in all people, but a majority of users of Propecia report satisfaction with the results.

Provillus is a hair care system that includes a minoxidil solution and dietary supplements containing vitamins and minerals. There is one version for men and another version for women. While improvement can take several months, many users report hair regrowth with Provillus.

Nioxin is a series of hair care systems sold in salons and online. Different products are combined to fight different types of hair thinning or loss. Nioxin works on the theory of cleansing the scalp to unplug blocked hair follicles and exfoliate the scalp, taking away DHT located on the scalp and potentially slowing hair loss while thickening new or existing hair.
Scalp Med is similar to Provillus for men. It comes with a minoxidil solution, a nutrient solution for the scalp, and a spray Cortex Enlarger that is a thickening hair spray.
Avacor is a hair care system that includes a minoxidil solution, two kinds of shampoo, Boost hair thickening serum and dietary supplements. The system for women has a different concentration of minoxidil, but also comes with scalp detoxifying shampoo, Boost thickening serum, and dietary supplements.
Procerin is a system of nutritional supplements and scalp conditioning foam. The supplements contain saw palmetto extracts, which have shown potential for blocking the conversion of testosterone to DHT. Saw palmetto as a DHT blocker is seen as a natural alternative to Propecia, a prescription DHT blocker that causes unpleasant side effects in a small minority of users.