Do a Google search. I just did for "how does google crawler work" minus the quotes and found a number of good answers from Google and other sites. No need to repeat it here.
the techniques and experiments behind this google tactics is Pegion technology. You can find more on these http://www.google.co.in/technology/pigeonrank.html
During the month, Google sends out robots (spiders) to crawl the web and archive every website it finds. These archived websites will be presented in the next update of the Google index (database). The update process is initiated every 30 days or so, although on some occasions there have been more than 30 days between updates.