In Ubuntu, there is no root user. Instead, it uses an authentication method named sudo. Sudo allows a normal admin user to do superuser tasks without becoming the superuser. Tasks that require superuser privledges include running programs that directly access hardware, starting and stopping system wide services and installing and removing programs.
The first user created (ie. you, who installed the system) is automatically an admin user and can run sudo. Other new users on your system will probably not have sudo privledges by default. If you want them to be able to use sudo, add them to the admin group in the Users and Groups management panel.
Please read a more indepth explanation of what sudo is, and how it is used on your system.
This is a quick summary I made for a class I was TAing for a couple of years ago. Better articles exist online, but the class needed a 3 line explanation, and this is what I gave them.