At the core of Mathematica is the foundational idea that everything—data, programs, formulas, graphics, documents—can be represented as symbolic expressions. And it is this unifying concept that underlies Mathematica's symbolic programming paradigm, and makes possible much of the unique power of the Mathematica language and system.
FullForm — the full form of an expression, without shortened syntax
expr/.rules — make replacements for any occurrence of a pattern in an expression
Cases — find occurrences of a pattern in an expression