Mathematica handles many different kinds of concepts: mathematical formulas, lists, and graphics, to name just a few. Although they often look very different, Mathematica represents all of these in one uniform way--as expressions.

This uniform expression model is a crucial unifying principle in Mathematica. The fact that every object in Mathematica has the same underlying structure makes it possible for Mathematica to cover so many areas with a comparatively small number of basic operations, and for each function (whether built-in or user defined) to be so powerful.

For example, there's no need for a special graphics manipulation language in Mathematica: the uniform expression model means that like every other kind of expression, graphics can be manipulated by "standard" expression manipulation functions. In a non-Mathematica system, graphics would have a different representation type from numerical quantities, so without special adaptation would need their own set of operations for manipulation.