Like a session, it is also a storage variable. I hope u are well favour with the session variable. But the only difference is viewstate is only with in that page. (i.e), if u used a viewstate["Test"], then "Test" is the variable that u can access only with in the page.
Using the viewstate is not adviceable because it increas the page wait. So if a application need to design with high speed and efficiency, using view state is not adviceable. In fact u can use hidden variable as a replacement.
Thanks.
Hope this may help u