AT&T Account login/password vs. Email accounts/passwords

I am trying to understand the different accounts and how they interact.  I go to ATT.com to log into my account using my email/password to check my internet or wireless service billing, etc..  I log into my email using my email address via my browser at mail.yahoo.com.  My email uses an AT&T email server and my email address has @sbcglobal.net.  This is all very confusing to me.

So, my question is... Is using my email address/password for logging into att.com to check account information the same as logging into my email with the same email address and password logical or should they be different?