Dan,
A "Refresh" simply calls the last request made for a page. So, if
you just browsed to a page then hit refresh, it simply asks for that page
again.
A Postback is essentially an action on the page that sends
information back to the server. This is done using what is known as the Post
method (there is also a Get method and that's what the variables in the
querystring really are, get fields). So any activity that sends the page
back to the server is a post. Normally this is done by clicking on a button,
link button, image button, or through client-side javascript.
Now, when you do a "Refresh" after some activity that caused a post,
such as pushing a submit button, the page will "Refresh" the last activity.
So, if you push the submit button, then hit refresh it will send all the
post information again.
ViewState is essentially a client-side state information store,
giving you a way to save values and have them held in the page across many
postbacks. This is required since the web is a stateless environment and the
web server doesn't stay connected to the client browser after the page has
been completely fetched. You can save or edit viewstate variables, and also
get their information. One of the key things to remember is the viewstate
can only really be changed when you post back to the server. Let's say you
have a counter viewstate property. You set it to 1 when the user first
visits the page. You would expect it to increment when you hit the refresh
button but it doesn't. The reason is all you're doing is refreshing the
first request and setting the viewstate to one again. When you hit a button
and do a postback you'll be in a position where that viewstate variable
already exists and is set to one so now it can be incremented to 2.
Another thing to learn about the viewstate is it's not available in all
page events. The viewstate gets loaded a little later in the page lifecycle
so that it wouldn't be available in the oninit event, but is available by
the time the pageload event happens.
