Whereas supervised and unsupervised learning appear at opposite ends of the spectrum, RL exists somewhere in the middle. It is not supervised learning, because the training data comes from the algorithm deciding between exploration and exploitation.
In addition, it is not unsupervised, because the algorithm receives feedback from the environment. As long as you are in a situation where performing an action in a state produces a reward, you can use reinforcement learning to discover a good sequence of actions to take the maximum expected rewards. The goal of an RL agent will be to maximize the total reward that it receives in the end. The third main sub-element is the value function.
While rewards determine an immediate desirability of states, values indicate the long-term desirability of states, taking into account the states that may follow...