A question about web security mechanism

From: Chenghuai Lu (lulu@cc.gatech.edu)
Date: 05/24/02

From: "Chenghuai Lu" <lulu@cc.gatech.edu>
Date: Fri, 24 May 2002 14:31:03 -0400

After I logged into my discover card account, I click refresh bottom in my
browser. My web brower show the dialog that "The page cannot be refreshed
without resending the information. Click Retry to send the information
again, or click Cancel to return to the page that you were trying to view.".
If I choose Retry, my information is sent and the page is reloaded. While
for cancel, the page cannot be reloaded.

My question is, what is the mechanism this website use for security? what
kind of information is re-sent? I assume that this is different from use of
session cookie since browser won't popup the dialog box when I refresh the
page in my yahoo mail account.

Does anyone know the answer?