"A representation is a formal system for making explicit certain entities or types of information, together with a specification of how the system does this...
This definition of a representation is quite general. For example, a representation for shape would be a formal scheme for describing some aspects of shape, together with rules that specify how the scheme is applied to any particular shape...
To say that something is a formal scheme means only that it is a set of symbols with rules for putting them together --- no more and no less...
Thus, there is a trade-off; any particular representation makes certain information explicit at the expense of information that is pushed into the background and may be quite hard to recover."
D. Marr, Vision, San Francisco: W. H. Freeman, 1982. pp. 20-21.