Cookies? Nope, no-one’s baking anything. Here’s the simplest way to describe a cookie:Imagine that you’re a first-time customer in a great big store. When you walk in, the greeter pats you invisibly on the back, and slips a business card in your front shirt pocket. The card has on it only the name of the store, and a unique number.
If you just look around, and then exit, the store doesn’t need to enter stuff on the card – but if you join the points club, enter a draw, buy with a credit card, or other stuff, the greeter may magically make a note on the card in your pocket, depending on their privacy policy and your own agreements.
This mean that your private info is kept on *your* computer, not on theirs.
The next time you come back, the greeter pats you on the back, and picks your pocket. It finds the card, (ignoring cookies form other stores and sites) and sees that you’ve been a gone for a while, and have shown interest in buying Science Fiction books. “Hi, Joan Smith.” it says “We have some new SF books since you’ve been here last.”
By far, most cookies are used for forces of good. It’s often convenient for you to have a site (where you’re a legitimate user) know who you are. Many cookies are used to stop repeat visitors from getting the same pop-up window again and again. Many membership sites, where you have to log in, can’t work without cookies, because without them, every new page will treat you like a first-time surfer.
Cookies can be set to evaporate after a certain time (days or years or minutes or when you hit the “log out” button).
There’s a potential for abuse here, certainly. But mostly they’re just useful.