Behavioural Targeting (or BT) is simply the ability to show you ads based on what you do (your behaviour) on the web.
The Wall St. Journal has an article discussing Microsofts attempt to use behavioral targeting in combination with search ads:
If someone types in "compare car prices" on Live Search, Microsoft's computers note that the person is probably considering buying a vehicle. The computers then check with the list of Hotmail accounts to see if they have any information on the person. ... Microsoft says that in testing in the U.S., behavioral targeting increased clicks on ads by as much as 76%.
Microsoft doesn't really matter much, especially in search. The biggest player in BT is Yahoo!, since they have the most email users and a broad network of content sites. Google could start to do it more as well, mostly based off of your search history, though they've mentioned they won't use it with AdSense. Tacoda is the best known, and probably biggest 3rd party ad network which touts BT more than anyone else.
Behavioural targeting is important because it works and when you have a huge database of peoples interests from their click stream and email, you can really improve ad response rate. In 2007, you should see a lot more offerings, a lot more proof that it works, and the price should go up a lot too!