Information Retrieval System Explained Using Text Mining!

While searching for things over internet, I always wondered, what kind of algorithms might be running behind these search engines which provide us with the most relevant information? How do they decide which result to show for which set of search keywords. This might be a no brainer for a few people, but definitely an interesting problem for some of the best brains around the world. To find the answer, I read every guide, tutorial, learning material that came my way. Information retrieval system is a network of algorithms, which facilitate the search of relevant data / documents as per the user requirement.