It is important to know that Linux is only the tip of an iceberg called "Free Software"2.5 or Open Source2.6. The particular characteristics of Open Source Software are as follows: they are redistributable at will and without cost, their source code is available, and it is modifiable by anyone sufficiently experienced in programming. This concept is used principally by GPL license systems (General Public License) although these are not the only ones. Apart from Linux, a gem of Open Source Software is called "Apache"2.7, which is a web server program for the Internet and represents 50% of the market share in its sector. The majority of software which serves to make the Internet work (at all levels) is Open Source Software.