In computer science, in particular networking, a session is a temporary and interactive information interchange between two or more communicating devices, or between a computer and user. A session is established at a certain point in time, and then torn down at some later point.
Session is server side request by the web browser for fast website surfing for every user.
Server side session:-
Server-side sessions are handy and efficient, but can become difficult to handle in conjunction with load-balancing/high-availability systems and are not usable at all in some embedded systems with no storage. The load-balancing problem can be solved by using shared storage or by applying forced peering between each client and a single server in the cluster, although this can compromise system efficiency and load distribution.
A method of using server-side sessions in systems without mass-storage is to reserve a portion of RAM for storage of session data. This method is applicable for servers with a limited number of clients (e.g. router or access point with infrequent or disallowed access to more than one client at a time).