The terms "Linux" (commonly pronounced /ˈlɪnəks/ in English; variants exist) and "GNU/Linux" refer to the family of Linux kernel-based operating systems – and not to any one operating system. Their base components, i.e. the Linux kernel (more precisely its System Call Interface (SCI)), the GNU C Library or the uClibc, the GNU Core Utilities and a couple of more packages, make many Linux operating systems behave "unix-like".
Linux is one of the most prominent examples of free software and open source development: typically all underlying source code can be freely modified, used, and redistributed by anyone.