I can log into the same account (https://stackoverflow.com/users/7751131/parvin) using:
- My Yahoo email address
- My Google email address,
- Google log in (the red bottom)
Why is that?
The Stack Exchange platform supports using several login-names/email addresses for the same account - more info can be found here.
Adding additional logins to your account is useful because it provides a way to access your account in case you lose your password. You can choose Google, Facebook, Yahoo!, and other popular OpenID providers; create a new Stack Exchange OpenID by providing your email address and creating a password; or you can manually enter another OpenID address.
Open up your user profile page (if you can't remember how to log in, go here and enter your email) and, on most sites, open the "Edit Profile and Settings" section of your profile, and then click the "My Logins" tab: my logins in new profile page You should see a list of your logins.