How Search Works? Learn in 3 Minutes from Google Expert

Learn how search engine works from the Google web spam head Matt cutts.

What happens when you do a web search. 

The first thing to understand is that when you do a Google search, you aren't you are actually searching the web, you're searching the Google's index of the web, or at least as much of it as we can find. We do this with the software programs called spiders. Spiders start by fetching a few web pages then they follow the links on those pages and fetch the pages they point to, and follow all the links on those pages and fetch the pages they link to and so on, until we've indexed a pretty big chunk of the web-many billions of pages stored across thousands of the machines.

Now, suppose I want to know how fast a cheetah can run. I type in my search, say, cheetah running speed and hit enter. Our software searches our index to find every page that includes those search terms. In this case, there are hundreds of thousands of possible results. How does Google decide which few documents I really want? By asking questions - more than 200 of them. Like, how many times does this page contain your keywords? Do the words appear in the Title, in the URL, directly adjacent? Does the page include synonyms for those words? Is this page from a quality website or is it low quality, even spamming? what is this page's PageRank?  That's a formula invented by our founders Larry page and Sergey Brin that rates a web page's importance by looking at how many outside links point to it, and how important those links are. Finally, we combine all of those factors together to produce each page's overall score and send you back your search result about half a second after you submit your search.

Source: https://www.youtube.com/watch?v=BNHR6IQJGZs