Linux is a Unix clone written from scratch by Linus Torvalds with assistance from a loosely-knit team of programmers across the Internet. It aims towards POSIX compliance.
It has all the features you would expect 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 was developed mainly to run on Intel x86 based PCs, using the hardware facilities of the 386 processor family (TSS segments etc) to implement these features. There are a number of successful ports to other hardware platforms. These platforms include DEC Alpha, PowerPC (Mac), Sparc, 68000. Linux is now one of the most ‘ported’ operating systems around. Linux is really becoming an open operating system.
One of the most important features of Linux is that the source code is freely distributable. Most of the kernel source code is copyrighted and covered with the GNU Public License. This effectively means that Linux is totally free.