It's the username and password you specify when you start your Mac.
However, on Mac OS X you can enable "Automatic Login" (System Preferences > Users & Groups > Login Options). In that case you don't have to enter your password if you start your Mac. You only have to specify the password at the time you enable automatic login or when you explicitly log out and log in again.
In this case your username is the name displayed in the left panel of "System Preferences > Users & Groups". The password is the one you entered at the time you enabled automatic login.
Last updated on March 15, 2012 by Chris