The Linux kernel is released under the GNU General Public License version 2 (GPLv2) plus proprietary licenses for some controversial BLOBs and is developed by contributors worldwide; Linux is one of the most prominent examples of Free / Open Source software.
The Linux kernel was initially conceived and created by Finnish software engineer Linus Torvalds in 1991. Early on, the MINIX community contributed code and ideas to the Linux kernel. At the time, the GNU Project had created many of the components required for a free software operating system, but its own kernel, GNU Hurd, was incomplete and unavailable. The BSD operating system had not yet freed itself from legal encumbrances. This meant that despite the limited functionality of the early versions, Linux rapidly accumulated developers and users who adopted code from those projects for use with the new operating system. Today the Linux kernel has received contributions from thousands of programmers.
=> Preemptive multitasking (both in user mode and kernel mode)
=> Virtual memory
=> Shared libraries
=> On Demand loading
=> Shared copy-on-write executables
=> Memory management
=> The Internet protocol suite and networking
=> Inter-Process Communication
=> Virtual File System and much more