inpro means in process , acutually sessions variables are 
by default store in inpro that means sessions variables are 
stored in current process, outproc means sessions variables 
are store in sql database.......

In Proc mode is usefull for small application which can be 
hosted on single server. This mode is most common and 
default method to store session information. session data 
is stored in memory of local web server. it is not ideal 
for web farms and web gardens
   Out of proc mode is ideal for scalable and highly 
avialble application. it supports web farm and web gardens. 
session state is held in process called aspnet_state.exe 
that runs as a windows service.