Heuristics and Biases are the ways human reasoning differs from a theoretical ideal agent, due to reasoning shortcuts that don't always work (heuristics) and systematic errors (biases).
See also: Affect Heuristic, Confirmation Bias, Fallacies, Rationality
The field of heuristics and biases was essentially created by Kahneman and Tversky in a series of experiments proving that people consistently make a set of errors when judging problems that have exact statistical answers. This isn't to say that humans only make errors on this type of problem; it's just a lot easier to track the errors they are making when you know an answer precisely. ...