Cookies - What are they and why you should care    Cookies are small pieces of text which are stored in your computer as you visit web sites.  A web site will place a cookie into your machine to help the website recognize you .

The Good Side...

This can be very useful (and necessary) when customizing a web site or accumulating items in a shopping basket. Without cookies, a  web site would have to treat every person as a "stranger", and would be unable to present a user with personalized content.  For example, if I choose to be an ongoing customer of Amazon.com, I consider it "pretty good marketing/customer service" that they can:

• greet me by name
• suggest new items which I should be interested in, based on previous purchases
• Remember the 3 items I've  placed into my shopping cart as I continue to explore their web site. Otherwise I would have to write down all the item numbers, and then enter all the information on one long order form when I am ready to buy.
• When I am ready to buy, I can simply enter a password instead of re-entering all my credit card and shipping information

Amazon is able to provide these features by writing and reading a line of text (cookie) on my machine that contains not much more information than: "amazon.com customer ID=some_long_unique_number"

The Bad Side...

The greatest concern with cookies is when they come from third party web sites.   For example, an advertising company may handle the advertising banners you see displayed on thousands of different web sites.  As you visit a web page, the advertisement banner (And its own cookie) may be downloaded from the ad_company's server.   It is possible for a major company such as Doubleclick/Google to develop an extremely detailed profile of your web surfing patterns including which sites you have visited, what key words have you searched for, etc. 

A well-written cookie will contain only obscure user IDs that make sense only to the web site that reads/write that cookie.  A poorly written cookie will contain your real name, real email address etc.  Also a word of caution, you may discover domain names in your cookies which suggest that your computer was used to visit "adult_only sites"  Your computer may or may not have actually visited such sites.   Many such cookies can be placed into your machine from the advertising banners from an "accidental page"  For example if a family member accidentally went to whitehouse.com instead of whitehouse.gov, they may have received many cookies from pornography banners.  (I just want to make sure I don't accidentally cause any divorces based on sneaky cookies being discovered in your computer)

• Cookiedemo.com - gives you your choice of cookie
• evercookie - Gives you a cookie in many locations..  See if you can really delete it.
• Ghostery - A browser plug-in that alerts users about the web bugs, ad networks and widgets on visited web pages
• Lightbeam - A Firefox add-no that graphically shows you all the hundreds of sites that are tracking you.
• FLASH COOKIES article: Think you deleted your cookies? think again.
• Adobe's page to control  Flash Privacy settings
• Wikipedia article about google analytics.
• How Internet Cookies Work - how stuff works - great explanation and actually shows you your cookie at their site - and how it is collecting information about your movement there.
• Wikipedia cookies article.
• Many marketing companies particpate in this opt-out mechanism. ( limited to only a handful of websites at a time)
• Stubborn cookies: Local Shared Objects, Evercookie, Canvas Fingerprinting. Verizon Mobile Supercookie, 
  The web never forgets,  and a list of 5000+ websites that used canvas fingerprints
• A description of google analytics.