Roughly speaking a computational model can be described as a numeric approximation of a situation or system instantiated in some form on a computational device. In practise this approximation will usually be a simplification incorporating the salient features of some real world physical or social situation upon which various functions or mappings are performed. These functions represent other phenomena or processes which are known to take place in the real world. These mappings are run on computers and, hopefully, we end up knowing something about how the real world phenomena interact - something more than we would have been able to find out with pen and paper. It is important to note that everything is represented (as all things on modern computers are) necessarily as a finite bit string, a point I will return to briefly (see Flake, 2001 and references therein for more on computation).