open a terminal and press the tab key twice.. This is common to all linux distros not only Ubuntu.. Though however the root user will have more commands than the normal user..
Yes, press tab twice, it will ask 'Display all 1688 possiblities ? (y/n) Type y (enter)
On the bottom of the page you will '--More--' that means that there are more pages
left. To move to the next page press the spacebar. You can find all the commands
executables in the '/usr/bin' and '/usr/sbin' directories.