Unix is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, development starting in the 1970s at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and others. It is written in C language and Assembly Language.It was started in 1969 and first published for use in 1973.Some unix systems are open source and some are proprietery.
Linux is a free and open-source software operating systems based on the Linux kernel. It was started by Linus Torvalds. It is said that he was furustrated by OS at that time, so he started linux and it got attention of many developer worldwide. It was released in 1971. It is mainly written in C language.Now , it is maintained by linus torvalds himself under organisation named linux foundation.
Linux is just a kernel which can utilize resources of a computer hardware and can interact with programs.
GNU is a Unix-like operating system. It is a collection of many programs: applications, libraries, developer tools, even games. The development of GNU, started in January 1984, is known as the GNU Project. Many of the programs in GNU are released under the auspices of the GNU Project called as GNU packages. GNU/Linux uses Linux kernel and GNU libraries , tools etc for OS. Many general people just donot mention GNU in GNU/Linux by mistake which takes credit away from it.
Don't get confused by unix and linux. Unix can be proprietry(HP-UX) and open source(openBSD) as no standard was set in earlier development of OS but linux has a central code base of development and standards are set by community. so it is open source and free software. Many different Unix was developed by different organisations for self usage purpose which cannot be used on ordinary systems.Linux is developed under POSIX standard. Linux and most unix can be used by other developers under licence of respective community.
Interesting usage about Linux kernel :
1. It is used on Desktops. It is available with different graphical user interfaces and tools.Ex -: GNOME,KDE,xfce etc.
2. It is used dominently on servers due to efficient resource management and long time working without reboot.
3. It is used in all top 500 supercomputers in world.
5. It is used in smart phones like andriod and in chromebooks as chrome Os.
6. It is now also available on windows as Window subsytem for linux. Major distributions like debian, ubuntu , kali, openSUSE, RedHat are available.