The root and user account are different accounts. You should never log in with the root account. You should always log in as a regular user and use sudo to elevate privileges temporarily to install programs or perform system maintenance. Actions performed by root are rather permanent, therefore you should use sudo so you think twice before executing a command.
Since the root and user account are different accounts, you will need to define two passwords. I strongly advice you to use different passwords for root and user accounts.
When you have selected automatic login, you shall need your password to perform actions that require superuser rights. For instance installing a new program, updating or altering configuration files.
Also have a read about this topic on serverfault: What's wrong with always being root?