What is the difference between SessionState and ViewState?

Session State contains information that is pertaining to a specific session (by a particular client/browser/machine) with the server. It's a way to track what the user is doing on the site.. across multiple pages...amid the statelessness of the Web. e.g. the contents of a particular user's shopping cart is session data. Cookies can be used for session state.
View State on the other hand is information specific to particular web page. It is stored in a hidden field so that it isn't visible to the user. It is used to maintain the user's illusion that the page remembers what he did on it the last time - dont give him a clean page every time he posts back. Check this page for more.

Session state is saved on the server, ViewState is saved in the page.

Session state is usually cleared after a period of inactivity from the user (no request happened containing the session id in the request cookies).

The view state is posted on subsequent post back in a hidden field.

SessionState

• Can be persisted in memory, which makes it a fast solution. Which means state cannot be shared in the Web Farm/Web Garden.
• Can be persisted in a Database, useful for Web Farms / Web Gardens.
• Is Cleared when the session dies - usually after 20min of inactivity.

ViewState

• Is sent back and forth between the server and client, taking up bandwidth.
• Has no expiration date.
• Is useful in a Web Farm / Web Garden