A cookie is a small piece of data that the server sends in a Set-Cookie header as a part of the response. The client stores cookies locally and includes them in any future requests that are sent to the server.

Servers use cookies in various ways. They can add a unique ID to them, which enables them to track a client as it accesses different areas of a site. They can store a login token, which will automatically log the client in, even if the client leaves the site and then accesses it later. They can also be used for storing the client's user preferences or snippets of personalizing information, and so on.

Cookies are necessary because the server has no other way of tracking a client between requests. HTTP is called a stateless protocol. It doesn't contain an explicit mechanism for a server to know for sure that two requests have come from the same client. Without cookies to allow the server to add some uniquely identifying information to the requests, things such as shopping carts...