Chaitanya Gupta has posted a nice introduction to reader macros. The conventional wisdom is that reader macros are ripe for abuse and many experts recommend avoiding them. Still, there are times when they solve a legitimate problem1 and in those cases they provide a powerful tool.
If you want a detailed explanation of reader macros, the best places to find it are still Paul Graham’s On Lisp and Doug Hoyte’s Let Over Lambda. Gupta’s tutorial is an excellent introduction and enough to allow you to write your own reader macros. If you’re a Lisper and don’t know about reader macros you should definitely spend a few minutes on Gupta’s tutorial.
Footnotes:
That is, a problem that doesn’t involve trying to make Lisp look like Python or something similar.