How google gather information about users to customize advertisemnet?

Rebel: How google gather information about users to customize advertisemnet?
From a technical point of view how Google in different ways can (or could) gather information about users and use it to customize advertisements, both at their own and others’ sites. What kinds of information can be gathered?

Answers and Views:

Answer by Nick
They can store ANYTHING that you submit to their servers directly or even indirectly through third-party services. They can also record pretty much everything about your computer, depending on what web browser and operating system you are using and what you have blocked. Even behind a proxy you are not completely protected. There is always a path back to you.

Answer by imisidro
The best way is to read and understand Google’s patent describing their contextual advertising process of scoring ads by price, relevancy, and performance, using a technique called “probabilistic hierarchical inferential learned cluster.”