He is a luchador as is his uncle, Rey Misterio, Sr. He debuted as a wrestler in 1991 in Tijuana, Mexico because he was too young to wrestle in the United States as no athletic commission would give him a license at his young age. There he feuded with another luchador, Psicosis (Psychosis) which carried over in to the states and lasted several years.
In 1995 he and Psicosis went to Extreme Championship Wrestling, making their first real impacts in American professional wrestling, outside of the WCW-promoted When Worlds Collide Pay Per View. Their fued stopped for a while after a Mexican Death Match in November 1995 at the ECW Arena in Philadelphia, PA.
In 1996, he joined Psicosis (now Psychosis) in World Championship Wrestling, continuing their feud on and off for a few years. Rey would find success there, winning the WCW World Cruiserweight Title on numerous occasions.
In 1998 he was involved in a feud with Kevin Nash which culminated with a Caballera vs. Mascara Match (Hair vs. Mask) which Rey lost. Now unmasked, he still found moderate success in WCW until it was sold to the WWF in 2001.
Rey then went back to Mexico to work for Consejo Mundial de Lucha Libre, unmasked, being involved in some big matches. In 2002, Rey was then signed by WWE, but was forced to wear the mask again, which is insulting and distasteful in lucha libre because once you lose the mask, you cannot regain it. In fairness, it wasn't Rey's call, though. In deference to this tradition, when WWE produced the Rey Mysterio: 619 DVD, Rey did not wear his mask during segments shot in Mexico.
Rey has made his mark in WWE already. He is a great aerial wrestler and a legend in the making.
References
http://www.divastudios.com/rey.html