Google has created a website that explains how they have indexed 30 trillion websites and they hit around 100 billion searches per month. The number 30 trillion is up 30 times in five years. Back in 2008, Google reported that they indexed one trillion pages. The Google Index amounts to about 100 million gigabytes (around 1,000 terabytes). This adds up to over 3 million 32GB USB thumb drives.
As you search, Google tries to guess what you are looking for using algorithms with spelling, autocompletion, etc. After Google determines what you want, it will pull the data from 30 trillion pages. Google uses a ranking procedure with over 200 secret factors including the quality of the website, domain age, safety, Google+ history, and location of the users.
Google also has human search evaluators checking quality also. The company notifies websites that they consider are spam or if they have been hacked at about 40,000-60,000 per month.