There are many topics not covered here as yet. Macros are, perhaps, the most important of these. I hope to write a brief introduction to macros at a later date as time permits. ``Macros'' allow one 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. As we know a lisp program is made up of series of expressions, macros are effectively a Lisp tool to write Lisp programs. Any further attempt to expand these ideas is likely to explode into a multiple page essay; so let's stop here. You should refer to the manuals distributed along with Emacs, in particular, The Elisp Reference Manual, if you cannot control your curiosity :)
Adios.