When you fill in the form required to obtain a browser, you provide your name and e-mail address to any site that is equipped to distribute cookies. Go to such a site, request a file, and the site will deliver your cookie along with an HTTP object. The cookie then follows you around the site, chronicling your visit. When you return to the site, the cookie's information goes back to the server. No server is supposed to be able to read another server's cookie, but inquiring minds are working hard to break that restriction.
Cookies do other things, as well.
If you're shopping, for example, they act as shopping carts, storing your selections until it's time to check out. That's a convenience to you, the shopper. But it also means that when you return to that site, the server knows what you've bought in the past and can advertise similar items to you. That's a convenience to you and to the cookie-passing site owner. B ut even names, addresses and purchasing patterns aren't enough for some site owners, particularly those seeking revenue through sponsorships.