When using a distribution that requires sudo to undertake any administrative tasks, the use of sudo is simple. Say you want to install Gnucash with apt-get. If you issue the command apt-get install gnucash you will see the error messages:
E: Could not open lock file /var/lib/dpkg/lock - open (13 Permission denied)
E: Unable to lock the administration directory (/var/lib/dpkg/), are you root?
The above simply means you do not have permission to do what you tried to do. Now if you prefaced the same command sudo like so: sudo apt-get install gnucash, you would see:
Once you enter your user password the command would execute.
After this command is completed, if you wanted to issue another command requiring sudo you wouldn't have to enter the password a second time. This feature does time out after five minutes of inactivity.
Those of you who are accustomed to Fedora, Mandriva, and SuSE will notice that, by default, you can not successfully issue the su command to gain root access. In Ubuntu there is no root password set by default so you can not log in as root. This system is added for security reasons.