Unix, the 'grand daddy' of Linux, has always been a MultiUser OperatingSystem. Because computers were very expensive, slow, and time on the typewriter was limited, people would write their programs and leave them running while the next user typed in theirs.
All users on a Unix system are the same, except one. That user is called the SuperUser and has complete control of the system. This user can kill any of the running processes and access all of the files (including the device files, so root is the one who configures the hardware), as opposed to ordinary users who can only mess with their own processes and files. Many other OperatingSystems have a similar notion of one or more super-privileged accounts. On Unix the SuperUser is called root  (think of a tree here), because this account alone has sufficient permissions to manipulate the files and directories involved in the creation of new user accounts. (You can change that of course, but you really don't want to.) If you are logged in as a regular user and you want to "become root", you use the command su(1) (which stands for switch user and can change you to any user).
Don't overuse the power of root! Working as root may be tempting since none of those pesky access restrictions boggle your mind, but it's very dangerous for exactly that reason. You accidentally rm(1) any part of the system and then it's gone. Someone who chances on your keyboard can get at anything on your computer. Any bugs in software can destroy your entire system. Nothing is safe from your touch.
 Extra for experts: the Linux SuperUser is defined by having UID 0, not by being named root. If you're particularly security conscious you can rename your superuser to anything you want, and your system will just keep on ticking. However, this is only SecurityByObscurity, so you should always have a strong and frequently changed password on your SuperUser account.
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