An aversive stimulus is something that someone tries to avoid. It suppresses a behavior by providing an event that the person wants to avoid. An example of this is a teenager coming home past curfew and in result gets their car taken away. The removal of the car is the aversive stimulus, which suppresses the behavior of coming home past curfew.
An appetitive stimulus is something that a person wants to approach. It is appeasing to the person and leads to reinforcement of a behavior. For example, while teaching a child math, the mother rewards him for extra correct answer with a cookie. By providing the appetitive stimulus as the cookie, the child will want to work hard to get correct answers in math.