Users
Within UNIX users are split into two general groups. There is the super user (root) who has total control over a system and normal users who have varying degrees of control. You can find out your user by doing "id". This will show you user id as well as the logical groups that you are in.
Groups allow you to be give extra permissions without having to give root access. So for example someone in the mail group might be allowed to edit the e-mail server configuration.
To switch between users you can use the su (switch user) command. If you are root it will let you become any user without prompting for a password. If you are not root you will be prompted for a password. By default running the command "su" will try to change a user into root.
You can tell quickly if you are logged in as root as your prompt will be a #. If you are not root it will be a $.