There are various kinds of hair loss, each with different causes and symptoms. The most common is male pattern baldness, a hereditary disorder which, in men, manifests in the receding of the hairline and thinning of the hair at the crown.
More random patches of hair loss called Alopecia Areata can also occur, in extreme cases leading to total loss of hair on the head or body. This is an immune reaction, and often hair will eventually grow back on its own.
There are treatments which will encourage hair to grow faster and stronger but there is currently no cure for hair loss, and any increased growth will recede if the course of medication is stopped.