MultiUser allows you to create a UNIX-like environment where several users live together in harmony, unable to delete each others files, unable to read those private love-letters of other users. And this even if several users are working on the machine at the same time (on a terminal hooked up to the serial port, or via a BBS or the internet)
People without a valid login ID and password won't be able to access files you have made private with MultiUser. If you make all files private (not readable for others), the only useful thing they could do, is boot from a floppy.
Multiuser was originally developed by Geert Uytterhoeven, but is currently being maintained and developed by Wez Furlong, Johan Eriksson and Richard Smith.
Read about Multiuser 2.