Think about the Web sites you visited recently. You shopped, checked airfare prices and researched a medical condition.

Marketers may very well have traded this information, along with your name and address. They could be tracking you with cookies.

HOW COOKIES WORK

A cookie is a small text file placed on your computer by a Web site. Some cookies are useful. They identify you to your favorite sites, so you can bypass site log-ins. The site also remembers your display preferences.

But advertisers and third parties also place cookies on your machine.

For instance, you may see an Acme Widgets ad on one site. The ad places a cookie on your computer, even if you don't click on the ad. Then you visit a second site running an Acme ad. This time, Acme reads the cookie on your machine. It knows the content of the first page you visited. It knows the content of the second page you visited. It can now offer an ad better targeted to your interests.

Now consider this: Acme ads are placed on sites by an advertising network. Goliath Advertising tracks and profiles you as you move from site to site, displaying ads targeted to you.

Many people find the targeted ads creepy and invasive. Well, that's not the half of it.

ONLINE MEETS OFFLINE

Data companies like Acxiom and Datran have gotten in on the game. These companies gather information from public records. They know the value of your home and your net worth. They know how many kids you have.

Now, imagine that offline data being used in conjunction with cookies. You might register on a Web site, giving your name and credit card for a purchase. A cookie tying your computer to your offline data might be set.

Targeted ads suddenly become more targeted. Retailers may customize prices to your income or buying habits: You end up being charged more than those who aren't targeted.

Even scarier, imagine researching an embarrassing medical condition online. A few days later, you find your home mailbox stuffed with information on products to cure the condition.

MURKY SITUATION

The flow of data between companies is murky at best. It isn't always clear who sets the cookies. Likewise, privacy policies are vague.

To stop the tracking, you must opt out. This is easier said than done. And deleting cookies may not help. Deleted cookies can be reconstructed.

Your best bet is to block all cookies, then create an approved list for sites you will allow to set cookies. This will be inconvenient at times. You'll need to add sites to the list occasionally.

In Firefox, click Tools, then Options and then Privacy. In the cookies section, deselect "Accept cookies from sites."

Next, click Exceptions. In the top box, enter addresses that should be allowed to set cookies. Click Allow after each one. When you're done, click Close, then OK.

In Internet Explorer, click Tools, then Internet Options. Open the Privacy tab and click Advanced. Select "Override automatic cookie handling." Select Block for first- and third-party cookies. Click OK.

Click Sites. In the top box, enter Web addresses that can set cookies. Click Allow after each one. Click OK, then OK.

One site you should approve is *.microsoft.com. Otherwise, you could have trouble installing updates for Windows, Office and other Microsoft software.