Frequently asked questions about HTTP cookies and persistent client state.
A Cookie (Persistent Client State HTTP Cookie) is sent through HTTP headers. It contains server-set information that's transferred by the client to the server with each request until expiration.
"Cookies are a general mechanism which server side connections (such as CGI scripts) can use to both store and retrieve information on the client side of the connection. The addition of a simple, persistent, client-side state significantly extends the capabilities of Web-based client/server applications."
Almost all browsers released after 1995 support cookies.