As part of the Setup Assistant that runs when you install Mac OS X or start up a new Mac, Mac OS X enables automatic log-in for the first user account that you create -- which means you can log in without providing a username and password whenever you start up.
While automatic log-in is convenient, particularly if you're the only user of your Mac, it also means that anyone who has physical access to your Mac can simply restart it to gain full access to your account and your files. This is a particularly significant risk for Mac laptop users.
You can disable automatic log-in in the Accounts pane of System Preferences by clicking the Login Options button at the bottom of the user accounts list on the left. The automatic log-in option appears at the top of the area on the right; select Disabled from its pop-up menu.