What's the difference between Linux and Unix?

Linux and Unix are not the same, but would you know how to explain the differences between them?

Unix is a propietary system which can't be modified, only AT&T, the company that created and owns it can do it. It was developed in the 1970s as an operating system to manage servers and nowadays it is mainly used for that purpose, except MacOS X, a desktop OS. Server operating systems are difficult to install, commands are more important than graphic interface and they are only compatible with specific hardware. AIS, HP-UX or Soralis are some Unix systems examples.

Linux Kernel was born in 1990s and it is based on Unix, but it is developed under GNU license, meaning that it is 100% free and the code is available to anyone and it can be freely modified. There are Linux operating systems for servers and clients, a wide variety of Linux distributions, desktop OS and tools. Red Hat or SUSE Linux are some examples of server systems and Debian, Ubuntu or Linux Mint are desktop OS.