The Unifying Idea of Mathematica
Mathematica is built on the powerful unifying idea that everything can be represented as a symbolic expression.
All symbolic expressions are built up from combinations of the basic form
A cell containing text
The uniformity of symbolic expressions makes it easy to add to Mathematica any construct you want.
All operations in Mathematica are ultimately transformations of symbolic expressions. Mathematica has a uniquely powerful pattern matcher for applying transformation rules.
Mathematica uses patterns to generalize the notion of functions.
An important feature of using patterns is that they allow "functions" to take arguments in any structure.