Data Brokers collect, store, analyze and sell consumer data with the aim of helping its clients -- including well-known banks, credit card issuers, insurance companies, department stores and carmakers -tailor marketing to their most valuable current customers or identify new customers.

Some terms relative to this are, "data mining", "behavioral tracking", micro-targeted advertising, See glossary

This can be good. Since you are going to get ads anyway, why not get ads for things you're interested in.
On the other hand you may not want ads for motorcycles showing up while you and your wife are looking for new garden furniture on the web, when your wife just told you to forget about a bike because they are too dangerous.

In A Data Broker Offers a Peek Behind the Curtain - NYTimes.com, Sept. 1, 2013, CEO, Scott Howe, says,
"Some federal regulators and privacy advocates warn that this kind of data-mining could be used to aim at consumers vulnerable to predatory lending practices, for instance, or to favor certain high-value consumers with instant, attentive customer service while relegating other people to interminable wait time.

Mr. Howe says he wants to counter such fears by making industry practices more transparent."

His company, Acxiom has a web site, AbouttheData.com, where you can see your data.

PersonicX segments U.S. households into one of 70 distinct clusters within 21 life stage groups.

What types of data do companies use?
Companies use lots of different kinds of data about you such as your age, whether you are married, single or divorced, what kind of vehicle you drive, whether you own your home or rent, etc. Generally speaking, data about you falls into two categories: core data and derived or modeled insights. Core data includes things like your address, phone number, age, etc. Derived and modeled insights are the result of analytical processes that use your core data to infer things about you such as whether or not you like sports cars or enjoy cooking.

Software:
AbouttheData.com shows what data Acxiom has about you.
See your Personicx cluster at isapps.acxiom.com/personicx/personicx.aspx
Collusion is a firefox add-on which shows a graph of sites tracking you when you are online.
  See Gary Kovacs: Tracking our online trackers | Video on TED.com.
See Privacy and SpyWare software here

Data Warehouses:

Phorm
NebuAd (out of business).

Glossary:

Behavioral tracking -
Behavioral targeting -
DPI - Deep Packet Inspection - Examining the data in a packet as it goes thru an
      inspection point, ISP, Firewall, ..
Data mining -
ENISA - European Network and Information Security Agency
Micro-targeted advertising

See SkyHorse.Org

Modern web browsers are subject to "device fingerprinting" via the version and configuration information that they will transmit to websites upon request. Much of this information is used to configure a response to your screen resolution and other paramaters than will improve the look of page.

But there are enough of these things that together they may be unique for your computer.

There are companies which sell software to fingerprint your computer.

Is your browser configuration rare or unique? If so, web sites may be able to track you, even if you limit or disable cookies.

Go to https://panopticlick.eff.org/