Multiple simultaneous users

Unix was designed with the notion that multiple people would be sharing use of the system at the same time. Several people can log into a Linux machine and each of them can run whatever programs they like. Because of the nature of the Linux GUIs (see the next detail) these users don't all have to be in front of the machine to fully use it.

Each user has their own individual preference files that allow them to cusomize their use of the system without affecting others. Unix applications are written from scratch with the idea that multiple people will be using them, and each may have entirely different preferences. One user of a mail-reader might prefer to read and send in full HTML, while another might want to read and send text-only. In Linux, they both can have their way, even if each of them is running the mail-reader at the same time!