Edi Weitz has a very nice set of notes on The Power of Lisp Macros. They're from a talk he gave at freiheit.com and are aimed at the beginning Lisper. They're amazingly easy to follow even though you aren't hearing the accompanying commentary. It's definitely worth a few minutes of your time.
Weitz is, of course, the author of much Common Lisp open source software including the indispensable CL-PPCRE that brings industrial strength Perl compatible regular expressions to Common Lisp. There's lots of stuff on his site so it's definitely worth a visit.