BenFox - 6:15 pm on Apr 16, 2011 (gmt 0)
I'm going to guess you're talking about ad re-targeting which shows you ads related to sites you've been on recently.
It works marvellously.
Alternatively there are companies that offer targeted display ads which are based on demographic information about your target customers. Can't remember what it's called and I've no idea if it works.