In complex problems, it is impossible to search the space exhaustively for the very best solution. Suppose, for example, you wear four different articles of clothing (i.e. shirt, trousers, socks and shoes) and you have 10 pieces of each article (10 shirts, 10 trousers, …). Then, there are 10.000 different combinations of clothes you can might wear each day, but no one has the time or interest to consider all the possibilities. Instead, people rely on heuristics, which are rules of thumb that contribute to satisfactory solutions without considering all the possibilities. A heuristic such as “wear brown shoes with brown trousers but not with black trousers” helps to provide an efficient solution to the problem of planning what to wear. Problem solving, learning, and language use can all be described in terms of rule-based heuristic search through a complex space of possibilities.
Source: Paul Thagard. “MIND: Introduction to Cognitive Science.” Second Edition, MIT Press, Cambridge, Massachussets.