Quoting from the Lisp Reference (a.k.a Lispref) Manual:
Macros enable you to define new control constructs and other language features. A macro is defined much like a function, but instead of telling how to compute a value, it tells how to compute another Lisp expression which will in turn compute the value. We call this expression the expansion of the macro.
Macros can do this because they operate on the unevaluated expressions for the arguments, not on the argument values as functions do. They can therefore construct an expansion containing these argument expressions or parts of them.
Do not confuse the two terms with keyboard macros, which are another matter, entirely. A keyboard macro is a key bound to several other keys. Refer to manual for details.