They usually contain a code that becomes your unique identifier, which lets a site know that you've been there before. Some cookies can contain personal information, such as a name or e-mail address, but only if you've given that information to the Web site. If you surf the Web, you likely have plenty of cookies on your hard drive. Probably the only sites that won't prompt your browser for cookies are personal home pages that don't carry ads and originate on local or regional Internet service provider servers. Cookies are often fewer than 100 bytes, so they won't affect your browsing speed.
Cookies perform myriad functions for both Web surfers and Web sites.
For the user, they make the Web more convenient. Personalized Web pages, such as My Yahoo, use cookies to customize the page with news, stock quotes, and other information that you indicate you want to see. Online stores use cookies to record purchases in electronic shopping carts before you leave the site. Those sites may also use cookies to help with order forms, so that the next time you buy something, the shipping and billing information gets filled in automatically. For businesses, cookies can play a role similar to that of a salesperson.