The symbolic language paradigm of Mathematica takes the concept of variables and functions to a new level. In Mathematica a variable can not only stand for a value, but can also be used purely symbolically. And building on Mathematica's powerful pattern language, "functions" can be defined not just to take arguments, but to transform a pattern with any structure.
x=... — set a variable
f[x_]:=... — define a function that takes any single argument
Clear — clear a function definition