What are cookies?
A cookie is a tiny bit of information that a web browser stores in your computer for a particular website you have visited.
Are they good or bad?
Cookies are benign at worst, and many are actively good. You may agree when you find out what they are used for.
Why do I need cookies?
Website communication has no "state", meaning that websites can't remember what has happened in the past. Cookies supply this ability to remember.
If you turn off your browser's ability to handle cookies, many websites will refuse to work properly.
What are cookies used for?
Cookies are used to remember what has happened in the past. They can do this because they are stored in your own computer.
For example, if you sign in to email and check the box saying "Remember me", then the next time you come back, you will automatically be signed in again without having to re-enter your address and password. This happens because your web browser gives the previously-stored cookie to the web server, and this cookie contains your credentials.
If you shop online, a cookie remembers what you have added to your shopping cart as you move from page to page, because the cookie is placed in your computer for storage.
Do cookies take up space in my computer?
Yes, but they are no more a storage concern than are the hairs on the floor of your house.
What happens if I delete them?
You will be immediately signed out of any websites that remember who you are. Because of this, I don't like to delete other people's cookies unless I need to for a technical reason.
If cookies remember my sign-ins, isn't that a security problem?
Yes, and so is your key ring. If you are the only one who has access to your computer, I am not concerned about anyone else getting your passwords or using your logins. If it's not your computer, then you should be concerned. Delete cookies when you are done.
I've heard tracking cookies are bad...
Web advertisers place cookies in your computer when they serve ads to you. These cookies may count the number of times the advertiser's ads have been served to your browser. Please read the LifeHacker article below if you are concerned; it goes a long way to dispelling exaggerated misconceptions about cookies.
When should I delete cookies?
Delete your cookies if you are having technical problems relating to signing in to a website, or if you need to reset your browser because of some technical problem.
Where can I learn more?
For more information, see:
LifeHacker's Fact and Fiction: The Truth About Browser Cookies
What is a Cookie?