What you see at google.com is a search engine. Essentially, search engines are software applications. Their main task is to search the internet for a given phrase. Search engines have specific algorithms for doing this. These algorithms have a way of deciding which sites should appear first in the search results (which sites are the most relevant).
The exact algorithms search engines use are not shared to the public. That’s why everybody involved with SEO can only guess what needs to be done to improve site rankings.
No surprise here, the biggest and most respected search engine is Google.