MacOS (up until osX, anyhow) was single-user and (technically) did not have multitasking capabilites (some people will try to tell you that the mac has “cooperative multitasking”, but I wouldn’t even cosider that multitasking, since it requires each running program to voluntarily pause and wait for others to run).
Windows, starting with Win95 is a single-user, multasking OS. Although newer versions liek XP and 2000 support file permissions via EFS, the DO NOT allow multiple users to be logged in at once.
UNIX is 100% multi-user and multitasking, and is therefore inherently stable, secure, and confusing. Once you go UNIX, though, you’ll never go back! UNIX (in all caps) was developed by AT&T waaaay back, Unix (no caps) usually refers to a family of operating systems (including the free-but-overrated Linux) that are based on UNIX. Solaris, BSD, OSF, SCO, and Tru64 are all flavors of Unix.