At the core of the Wolfram Language 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 the Wolfram Language's symbolic programming paradigm, and makes possible much of the unique power of the Wolfram Language and the Wolfram 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