It's a program that Yahoo runs that searches the Net for content for their search engines. Google, MSN etc do the same. So when you use a search engine there is a good chance it will find relavent forum content.
Don't listen to Ian. He doesn't know anything about it.
Actually, Yahoo is starting a new online smoothie finding/ordering company. You type in what flavor and type of smoothie you are craving at the time and it does a search and finds the nearest store, using Yahoo Maps, orders it for you and then you pick it up already made. Or you can have it delivered to you.
Ian nailed it. I flipped a switch on the forum recently to cause the search bots to be identified correctly, instead of as "Guest". We're likely to see more spiders around here than we used to, now that they are being identified correctly. In this case, they are friendly spiders.