Basically a wrestler.
Um...no, although it does involve some wrestling. "Cage fighting" also known as "Ultimate fighting" is Mixed Martial Arts (MMA)-- any style of fighting can be used in order to achieve a knock out or "tap out" (the latter is where one fighter is on the ground, usually in a choke hold, and taps the mat to give up), but usually it's a mixture of some kind of striking method (muay thai, boxing, karate, etc.) and some kind of grappling/throwing/wrestling method (jiu-jutsu, Brazilian or Gracie jiu-jitsu/BJJ, judo, sambo, etc.). There are fewer rules than in most other combat sports-- usually fishooking/eyegouging, small joint manipulation, head-butts and groin strikes are banned, but anything else goes-- in the early UFC fights here in the US, only eyegouging was banned, that changed when law makers started threatening to ban the sport entirely in the mid to late 1990s.
So basically a cage fighter is a wrestler/boxer or, more commonly, kickboxer. Some concentrate more on the groundwork (wrestling/grappling) and some concentrate more on striking while standing up.