Quote from Runningbear:
Google uses web crawlers or sometimes called spyders that go out and find new pages and data. It effectively copies this information and keeps it on 500,000 low cost servers. So when you do a web search, google actually searches for the information on its own database.
The google algorythm is designed to return to sites and update the data more frequently if sites continually add new information. So if you have a static web page, the spyders will return say once a month. If you have a news site, they may return once a minute.