As there are four major sanctioning bodies, and each has its own rankings of contenders and champions, it’s possible to have four different world champions at each weight division. The four belt era has been around since 2004.
A boxer can hold championship belts in one or more. A unified champion is one that holds two or more belts at once. If a boxer holds all four belts at once, they are known as the undisputed champion at that weight.
Boxing Unboxed 