Session: Its nothing but defined as a period of time shared between the web application and user. Every user has individual session. Items/Objects can be placed into the Session which would only define these object for that user. Session contains key variables which help to identify the related values. This can be thought of as a hash table. Each user would represent a different key node in the hash identifying unique values. The Session variables will be clear by the application which can clear it, as well as through the timeout property in the web config file. Usually the timeout is 20 minutes by default.
Session Variables are stored on the server, can hold any type of data including references, they are similar to global variables in a windows application and use HTTP cookies to store a key with which to locate user's session variables.
The collection of session variables is indexed by the name of the variable or by an integer index. Session variables are created by referring to the session variable by name. You do not have to declare a session variable or explicitly add it to the collection.
Lets get it cleared from following example:
Session["firstName"] = "FName" //User's first name
Session["lastName"] = "LName" //User's last name
// Clear the session variable
Session["FirstName"] = null;
//Clear all Session variables
//To Set the Null value it will not kill the session but assign the value to null
Session["firstName"] = null;