There are 3 types of session modes available in asp.net
1)Inproc
2)State Server
3)SQL Server

The following list describes the available session state 
modes:

1) Inproc mode
2) Stateserver mode
3) Sqlserver mode
4) Custome mode---> This enables you to specify the custom 
storage provider
5) Off mode---> It disables session state.

(1) InProc Mode or InProceess this is the clarification of 
InProc.
(2) SQL Server Mode
(3) StateServer Mode -->StateServer is memory based mode. 
(4) Out of process Mode -->This mode is best used when 
performance is important but you can't guarantee which 
server a user will request an application from.
(4) Off Mode -->Disable Session State.

there are two types of session state
1.in process
2.out process

in process we preserve the sesion state on the server itself
and in out process we store the session state in some 
external storage lke sql serer

in this two type of session state management
Those are :
     1)In-Proc
     2)Out-Proc
In the In-proc:
        we have in-proc model
In the Out-Proc
1)State server session state
2)Sql server session state
3)Custom server session state
and aslso
mode="off"-->in this session will be disabled