It's basically a program that logs onto the site periodically so that it can gather information for Google.com about this forum. That information is then used when people do Google searches for a particular subject.
MMiz has signed us up for a couple of these. I believe the other one is Archive.org.
GoogleBot is a bot that "spiders" the site, or goes through all the links. This is how google has all the amazing hits when you search for obscure search strings.
GoogleBot and MSN's new bot are the ones that seem to search the site the most. GoogleBot is here daily, and so is MSNs.