Linux (or more correctly GNU/Linux) is an open-source operating system released under the GPL that is especially popular on servers. Originally written by Linus Torvaalds in 1991, Linux is the name of the kernel component of the system.
GNU/Linux distributions include the GNU user-space tools produced by the free software foundation, plus a range of other software written by third parties. A number of highly customised variants are also produced, of which the best known is Google's Android.
Many database management systems will run on Linux, although Microsoft's sql-server is a notable exception.