Is Mac OS X a single-user operating system?

Answer 1

No. OS X supports the separation of user privileges and files. It follows the same semantics as other Unix and Unix-like systems. A single, all-powerful user known as root can do everything. Root can give the ability to, say, install software or connect to the internet to another user. Most users using OS X are running as an unprivileged or underprivileged user, and use "sudo" to make changes to the system without logging in or operating as root.