Managing session,states in asp(how ,where .why) !

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C#3.0

C#

ASP

ASP.NET

I am quit confuse about session in asp.I am using it but my concept about session is not clear .let supposed I have a piece of code here

if (Session["UserId"] == null)
            {
        Response.Redirect("~/Pages/Logout.aspx");
            }
            if (!Page.IsPostBack)
            {
                binlabel = "Lable";
                MyFunction();
            }

It says there is a value in "UserID" its checking if its vale is not null,but what value it has for how long it has that value.? how can we mange session for diff-2 controls.Quit confuse..Please help me out in this.I will really appreciated for your help.
Thanks..