Google Site links are updated periodically and it is a process in which the Natural listings in Google SERPs are shown as tree structure to the internal pages of a site. For example type "webopedia" in google you will find the listings like this one:I had found that the keyword that is mostly targetted in the client's site doesnt show the sitelinks on SERPs, whereas the term that is not targetted much, but a competitive threw site links on Google SERPs. Infact, the competitors though they are best of the sites are not listed for that keyword.
Google published a patent application Last week which gives an insight into the process of Google Sitelinks. Systems and methods for providing search results
Invented by Luis Castro, Walt Lin, and Benedict Gomes
US Patent Application 20060287985
Published on Friday, December 21, 2006
Filed June 20, 2005
An abstract of that Patent: "A method includes generating search results in response to a user query, where at least one of the search results includes a group of links. The group of links may represent links to web pages within a same web site and may be identified based on at least one factor associated with the links. The method may also include providing the search results to the user."
On conclusion, I would say that the process is purely inclined upon the way the pages navigated and are linked to internal and external sites. Google site links are discussed at many websites and forums.
However Google officially says in Webmaster tools that "Sitelinks are additional links Google sometimes generates from site contents in order to help users navigate your site. Google generates these sitelinks periodically from your site's contents. Because we generate sitelinks dynamically, this list can change from time to time".
