Information about browser cookies on your computer

I’ve had a few people asking me about cookies recently, so I thought I’d do a quick entry here to get some answers out there.

What is a cookie?
A cookie is a small piece of information that a web site asks your computer to store for it. If you have a cookie stored for a particular website and it hasn’t expired, your browser sends that cookie to the website with each request.
Here’s the information stored in a cookie I just copied from my computer:

Name: PREF
Content: ID=cbfa8e97b6fa36d5:TM=1163021506:LM=1163021506:S=ZOlLlGVj5ialaPx3
Domain: .google.com
Path: /
Send For: Any type of connection
Expires: 01/17/2038 12:14:07 PM

Do cookies contain private or personal information?
Yes, some cookies may have sensitive information in them. For example, when you log into your bank’s website, there is almost always a temporary cookie that is stored to keep you logged in as you go from page to page. Every website like that sets cookies that expire when you exit the browser or within 10-15 minutes of inactivity. While it is possible for cookies to contain private or personal information, only the website that the cookie belongs to can create a cookie or read it. The cookie above will never be read by any website unless it is a google.com website.

How long do cookies stay on my computer?
Some cookies have an expiration date and some are considered session cookies. Session cookies are deleted when you exit your web browser. The website that sets the cookie tells your browser how long to keep it. In the example above, that cookie will be kept for 32 years (unless I clear my cookies – see below.)

How/why are cookies used?
Here are three reasons why sites would want to use cookies:
Settings – Most websites use cookies to store settings you have chosen. For example, go to yahoo.com and enter in your zip code for the weather section. The next time you go there, it will “remember” your zip code. This is done with a cookie.
Shopping – Websites will set a cookie that identifies you and link that to items you have added to your shopping cart while browsing their site. They store this information in a database and may even link it to previous purchases you’ve made with them.
Tracking – To improve websites, they will use cookies to track how you use a website and hopefully make it easier to use. Cookies are also used to track referring sales.

Are cookies dangerous?
Cookies are not dangerous. Cookies are stored so that no website has access to others’ cookies in your browser.

Do cookies slow down my computer?
The cookie I listed above for google.com adds 81 bytes to web requests and takes up less than 180 bytes of storage on your computer. To give you an idea of how small these are, it would take about 1/100th of a second to upload that cookie over a dialup connection and you could fit over 4 million of them on a single CD. So technically, yes they do slow down your computer but you will never notice it.

How do I view or remove cookies from my computer?
Cookies are not shared between browsers on the same computer so you may need to look in multiple places to find them.

Firefox: In “Preferences” or “Options”, go to the “Privacy” section under the “Cookies” tab.
Internet Explorer (WinXP): Cookies are stored in “C:\Documents and Settings\YourUsername\Cookies” and can be deleted by going to “Internet Options” in Internet Explorer on the “General” tab and clicking the “Delete Cookies” button.