Configuring session state: Session state features can be 
configured via the <sessionState> section in a web.config 
file. To double the default timeout of 20 minutes, you can 
add the following to the web.config file of an application:
<sessionState
timeout="40"
/>

SessionState tag is used to define session attributes.
One very important is Mode which can have values: "state 
server", "SqlServer", "InProc"; depending how you want to 
preserve session state.
"State server" is used if you want the state to be 
preserved in other server.
"SqlServer" is used if you want the state to be preserved 
on a SQL database.
"InProc" is used if you want the state to be preserved in 
the same process where the asp.net page is running.

Other attributes that are also included for the 
SessionState tag are: ConnectionString, 
SqLConnectionString, userId, password, cookiless, tiemout.

 if the session state is going to be preserved in other 
server, or can be ""SqlServer" if session state is going to 
be preserved in a SQL database