what does forms authentication protect, as opposed to using session variable

I'm working on an application which uses the Session variable to keep track of users, checking on the master page for it's existence otherwise knocking them out to login. I wanted to change this over to Form Authentication as I read it was more secure and the data is encrypted.

Can someone tell me what data is actually encrypted? I tried setting up Forms Authentication on my site, it works fine, users are being tracked properly and can't access pages without logging in. However, when I look at the Request Body, using Fiddler, I see all the forms fields and there content. Can't a hacker use that to change the data and resubmit the request, like they would with a cookie generated from a Session variable? This application is not using SSL, so I understand SSL would encrypt the body, but I thought that's what Forms Authentication would do also. Otherwise what does it encrypt, just the Session ID in the cookie?

Here is the code I was using:

    <authentication mode="Forms">
  <forms loginUrl="default.aspx" name=".ASPXFORMSAUTH_Test" defaultUrl="home.aspx" protection="All"/>
</authentication>
<authorization>
  <deny users="?"/>
</authorization>

in the login page I tried to manually create the cookie:

                    FormsAuthenticationTicket ticket = new FormsAuthenticationTicket(1,
                    txtEmail.Text,
                    DateTime.Now,
                    DateTime.Now.AddMinutes(30),
                    false,
                    txtEmail.Text,
                    FormsAuthentication.FormsCookiePath);

                // Encrypt the ticket.
                string encTicket = FormsAuthentication.Encrypt(ticket);

                // Create the cookie.
                Response.Cookies.Add(new HttpCookie(FormsAuthentication.FormsCookieName, encTicket));

                // Redirect back to original URL.
                Response.Redirect(FormsAuthen