Session Declaration is all follow
{
Session[“UserName”]=”Vaibhav”;
}

Use declared session like
{
string Name =(string)Session[“UserName”];
}

Declare :
Session[“userId”]=”1”;
Retrive:
string userId =(string)Session[“userId”];

Session is a cookie variable in server and it is handle in server level in our entire web technologies (asp.net,asp.net mvc).

1. in asp.net

   System.web.HttpContext httpcontext= new System.web.HttpContext();
   httpcontext.Current.Requiest.Session["variable_name"]="variable_value";

2. in Asp.net MVC

   Session["variable_name"]="Session value";

   I hope above is ok our understanding purpose.