Linux is a Unix clone written from scratch by Linus Torvalds with assistance from a loosely-knit team of hackers across the Net. It aims towards POSIX compliance. It has all the features expected in a modern fully-fledged Unix, including true multitasking, virtual memory, shared libraries, demand loading, shared copy-on-write executables, proper memory management and TCP/IP networking.
It runs mainly on 386/486/586-based PCs, using the hardware facilities of the 386-processor family to implement these features. Ports to other architectures are underway.
Linux is freely available, and no one is required to register their copies with any central authority, so it is difficult to know how many people use Linux. Several businesses are now surviving solely on selling and supporting Linux, and very few Linux users use those businesses, relatively speaking, and the Linux newsgroups are some of the most heavily read on the internet, so the number is likely in the hundreds of thousands, but hard numbers are hard to come by.
Multitasking. Several programs running at once.
Multiuser. Several users on the same machine at once. Two-user licenses are not possible.
Multiplatform. Runs on many different CPUs, not just Intel.
Multiprocessor. SMP support is available on the Intel and SPARC platforms (with work currently in progress on other platforms), and Linux is used in several loosely-coupled MP applications, including Beowulf systems and the Fujitsu AP1000+ SPARC-based supercomputer.
Runs in protected mode on the 386.
Memory protection between processes. So that one program can't bring the whole system down.
Demand loads executables. Linux only reads from disk those parts of a program that are actually used.
Shared copy-on-write pages among executables. This means that multiple process can use the same memory to run in. When one tries to write to that memory, that page (4KB piece of memory) is c...