The symbolic language paradigm of the Wolfram Language takes the concept of variables and functions to a new level. In the Wolfram Language a variable can not only stand for a value, but can also be used purely symbolically. And building on the Wolfram Language'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