HyperText Transfer Protocol (HTTP) is a stateless protocol. However most of the web applications we use nowadays needs to maintain a certain state, as otherwise we would have to login each time we request a new page.
There are several techniques a web developer can use to simulate a stateful experience.
Sessions
To maintain a sense a statefulness, web servers can send a unique token that is sent back by the user’s browser. This token is referred to as session identifier.
Technically, each request is still stateless and do not know about the previous one, but the session id creates the illusion of a persistent connection between requests.
The server has to do some work to simulate statefulness. In particular, it has to parse the session id, check it it still valid (for security reason) and craft a personalized response for this request.
HTTP Cookies
HTTP Cookies are small files stored in the browser, that contain the session information. The session data is always stored on the server: cookies are used to compare what the client has with what the server has and make sure, for example, that the session hasn’t been forged. For this reason the cookies’ data must remain secret.
ID sent with session are (supposed to be) unique and expires after a short period of time. When a session expires, one needs to login again. The session also (is supposed to) expires when one log out.
AJAX
AJAX is short for Asynchronous Javascript and XML. This technology lets you send and receive requests without a full page refresh. All requests are performed asynchronously or, in simple term, without a page reload/refresh.
Responses from AJAX requests are processed by callbacks. A callback is a logic code passed to function to be executed after a certain event has happened; in this case, when the response is returned.