The word model is used in various contexts meaning something (abstract or physical) that represents "the Real World". That entity may be anything from a single item or object (for example, a bolt) to a complete system of any size (for example, the Solar System). In general, a model is an object which we study, not for its intrinsic interest, but because it is a formalized or simplified representation of a class of phenomena which can be studied easily... Typically a model will contain only the significant features or aspects of the item in question, and two models of the same item may differ quite significantly. This may be due to differing requirements of the model's end user (one user may be interested in aspects of the item which are quite separate from those of another user) or, perhaps more simply, this may be due to the difference in perception of that item by the modeller and decisions made during the modelling process. http://www.wikipedia.org/wiki/Model
All models are wrong. Some are useful.
evaluating a model
beware Model Theism