Main results

Own database, collected and maintained by spiders. They maintain several databases, the main worldwide one accessed through the www.google.com site, plus regional databases more focused on local sites, (ie www.google.co.uk contains less sites overall, but the contents are more UK focused. There are equivalent www.google.*** sites for just about every country in the world). There is some movement of entries back and forth between the regional and main .com databases.

Images

Their own database, again collected and maintained by spiders.

Directory

The Google Directory is based on the Open Directory Project. Sites included in the ODP will end up in the Google Directory after a few months delay as data travels from the ODP to Google. Google additionally apply their own filters to this data, so there are some differences between the ordering of results on the two.

Groups

Google acquired several years worth of Usenet postings (Usenet is a worldwide network of newsgroups, covering virtually every subject imaginable. They share a hierarchical naming convention, such as alt.humor.parodies or rec.parks), and has made them available. Particularly useful for research on technical subjects.

News

The most recent addition to the main Google search offering is the "News" service, which crawls thousands of online news sources, ranging from the big agencies to small local publications with an online component, and a handful of purely online publications