Wikipedia also has an article on this topic.
5α-Reductase (5-AR) is the enzyme in the cytosol of cells that converts Testosterone into Dihydrotestosterone (DHT), which is ~100x more potent. However, this isn't 5-AR's only job - it also metabolizes neurosteroids in the brain, and in general reduces (replaces Oxygen with Hydrogen) on the 5α position on any steroid.
5-AR exists in three isozymes, i.e. there are three different genes that produce the three different forms of the 5-AR enzyme: SRD5A1 (Type I), SRD5A2 (Type II) and SRD5A3 (Type III). Each of these enzymes behaves subtly differently, and not all drugs target all three isozymes equally.
- 5-AR inhibition may cause a form of androgen deficiency. 
Finasteride is the most common 5-AR inhibitor. Officially marketed for androgenic alopecia ("male-pattern baldness"), it's inexpensive, easy to obtain and generally trusted by clinicians.
- Long track record of clinical use.