InProc, state mantain in the memory of the ASP.NET process 

And OutProc, session  managed by an external resource (

Like StateServer or SQL Server 

and Inproc session states is faster than outproc 

Inproc not need to be serialized 

some times we also use Caching Technique to manage the State in outproc medium like M-Cache N-Cache but its use in Large enterprise applications