This is a stimulus that naturally triggers a response without any learning needed. For example, the pain from touching a hot stove is an unconditioned stimulus.
This involves removing an aversive event or stimulus to increase the likelihood of a behavior happening again. For instance, turning off an annoying alarm clock (removing the aversive event) when you wake up on time (desired behavior).
This refers to adding an aversive event or taking away something desirable after a behavior occurs with the aim of decreasing that behavior. For example, getting grounded (adding an aversive event) for coming home past curfew (behavior).