A cage fighter is someone who fights in mixed martial arts. Mixed martial arts consists of boxing, wrestling, muay thai, judo, taekwondo, etc. They are called cage fighters because they often fight in caged rings or octagons. Being an art teacher by day and a cage fighter by night means that his day job is an art teacher, but after school he trains as a fighter. Here is a link that explains it better: http://en.wikipedia.org/wiki/Mixed_martial_arts

A cage fighter usually refers to someone who competes in mixed martial arts competitions. This can be a traditional ring, like boxing, or, with the popularity of the UFC growing here in the States, it is more often a hexagon shaped 6 foot high fence. Here's an example of amateur cage fighting in a round "cage". http://www.youtube.com/watch?v=YKQorsdqmoA