Bishop best describes reinforcement learning in a single concise sentence: "Reinforcement learning is concerned with the problem of finding suitable actions to take in a given situation in order to maximize a reward." 1 Reinforcement algorithms are not given explicit goals; instead, they are forced to learn these optimal goals by trial and error. Think of the classic Mario Bros. video game; reinforcement learning algorithms would, by trial and error, determine that certain movements and button pushes would advance the player's standing in the game, and trial and error would aim to result in an optimal state of game play.
Matthew, Mayo. “Machine Learning Key Terms, Explained.” KDnuggets, KDnuggets, 10AD, 2016, https://www.kdnuggets.com/2016/05/machine-learning-key-terms-explained.html (1)