Fundamental but difficult to define, the concept of "concept" was discussed for hundreds of years by philosophers before it became a focus of psychological study. Concepts enable humans and animals to organize the world into functional groups; the groups may be composed of perceptually similar objects or events, diverse things that have a common function, relationships such as same versus different, or relations among relations such as analogies.  Extensive discussions on these matters together with many references may be found in Shettleworth (2010)  Wasserman and Zentall (2006)  and in Zentall et al. (2008). The latter is freely available online.