HowtoForge have created a guide to Killing greylisting and other spamkilling with Postgrey and Postfix
Read about it here
Greylisting in short means that when someone wants to deliver a mail to your mailserver it will simply reply Please come back later. That is something all RFC compliant mailservers do and when they do come back the mail is accepted. Most spammers and spam software are not compliant and not patient enough to try again. You will be surprised to see how effective this is. Anyway, follow the links below to really learn about it. There are as always pros and cons so do your homework before you put it on a production server.
Greylisting on Wikipedia
There are several implementations of greylisting and it can be done with most any server. I will show you how to do it on a Debian Sarge with Postfix running as MTA with Postgrey.