A theme is defined by search engines as a common topic throughout the site. In an effort to provide their users with more relevant information, search engines developed sophisticated technology that "extracts" site's themes. This technology allows results to be more focused on the topic searched for. Hint: Think of themes as your most important keywords.
Themes technology is quite complex. It's used by some of the major search engines when indexing and ranking sites. Currently, Excite, Altavista, Lycos and Google use themes as part of their formulas. Inktomi will soon join them. That will include AOL, Hotbot, iWon, LookSmart, MSN and NBCi.
The purpose of themes is to look at the bigger picture, not just the parts. To determine a theme spiders index the whole site and then analyze its HTML. Factors analyzed are: HTML title, meta tags, headings, text on the site, and links. Spiders use technologies such as keyword weighting, extrapolation and root stemming to determine a theme.
Excite, Altavista, Lycos and Google use them
Inktomi should be joining soon
Determined by keywords in title, meta tags, text and links