Why don't you just check the help docs?
"In Maya, a character set is a node that brings together into a set all the attributes of any collection of objects that you want to animate together. The character set could be anything: a well-armed robot, an automobile, or even some seemingly unrelated collection of objects. Maya enables you to bring together all the attributes together in a character node, so you only have to select one node, the character node, when you want to animate all the various attributes."
Another good use of character set is you can create clips of your animations, reuse them later or even on another characters with Trax. Subcharacter set is very useful too.