What is the roles of karma in Buddhism?

Karma in Buddhism is slightly different than most people think.

Karma is not the result that you get from an action (what goes around, comes around) as most people think. Karma is your action / reaction to a situation. When speaking about your Karma, we are speaking about the typical reactions you have to the same stimulus (like getting angry when someone tell you something you didn't want to hear).

The results of your karma (actions) is called the karmic fruit. That's what the "what goes around, comes around" is.