Googlebot, Yahoo Slurp, and MSNbot and similar spiders, bots, and crawlers are the programs that harvest information for search engines.
For anyone tracking statistics on their website, Googlebot, MSNbot, and Yahoo Slurp can be welcomed guests. These three search engine bots gather (harvest) information about your page for their respective search engine. Seeing these spiders more often is also desirable because this means that you are being indexed more often and more likely to show up quickly in the SERPs (search engine results page).