Positive reinforcers are stimuli that are added to the environment following a behavior, and their addition increases the likelihood of that behavior. Examples include praise, treats, or a desired object. If a child cleans their room and receives a sticker (the added stimulus), they are more likely to clean their room again.
Negative reinforcers are stimuli that are removed from the environment following a behavior, and their removal increases the likelihood of that behavior. This is often misunderstood as punishment, but it is distinct. For instance, if a person takes an aspirin (the behavior) to relieve a headache (the aversive stimulus), the removal of the headache (the stimulus) reinforces the behavior of taking aspirin. The individual is more likely to take aspirin when they have a headache in the future.
The effectiveness of a reinforcer depends on several factors, including its immediacy, size, and the individual's motivational state. What serves as a reinforcer for one person or animal may not for another. The systematic application of reinforcers is a key technique in behavior modification and training.