The sudo command allows you to perform actions as root but logs the actions you take (so you can trace anything that was done to the system by yourself or others). sudo has a very good manual page which provides plenty of information about it.
You use sudo similar to how you execute a normal command with sudo prepended to it, for example:sudo rpm -U myrpm.i386.rpm
This would allow you to install a rpm even if you have the correct sudo access
Simply typing su will give you some root privileges, but there are minor complications relating to environment variables. It's generally considered better practice to use su - because it has no restrictions.
The superuser. This user has power over everything and all, and can do anything with the system (including destroy it, and of course fix it :)). This user is used to perform most administration functions on the system.
If you need to edit either file it is recommended that you use vipw to edit the password file and vigr to edit the group file. These particular commands take care of any processing and locking of the files before and after editing them.
Use the -f option to change a users full name. Use this tool as either chfn or chfn user_name (usable by root only).