View State are valid mainly during postbacks and 
information is stored in client only. Viewstate are valid 
for serializable data only. Moreover Viewstate are not 
secured as data is exposed to client. although we can 
configure page directive and machine key to make view state 
encrypted. Where in case of session this is user specific 
data that is stored in server memory . Session state is 
valid for any type of objects. We can take help of session 
through different web pages also.