While many people know that Google offers a variety of web services, they may not understand how Google earns money. Since almost all of Google’s services are offered for free, Google looks for alternative sources of income. In many cases, Google is able to earn money through data mining and advertising revenue.
When an individual loads his or her Gmail account, a small bar will appear on the right side of the screen. This screen contains advertisements that are related to the content of an email. While an individual’s emails aren’t read by Google, they are analyzed by a special computer algorithm. This algorithm is used to determine what products and services an individual is most likely to buy. For example, loading an email with information about a flight might lead to a list of advertisements related to traveling or luggage.
There’s no way to disable data mining in Google’s services. However, many people are willing to sacrifice their privacy for the convenience of Google.
Source: Best Computer Science Schools