Git is a distributed version of files control system. The project was created by Linus Torvalds for managing of development of the kernel Linux. The first version was released in April 7, 2005. Nowadays, it is supported by Junio Hamano. Examples of projects that use Git, are the core of Linux, Android, Drupal, Cairo, GNU Core Utilities, Mesa, Wine, Chromium, Compiz Fusion, FlightGear, jQuery, PHP, NASM, MediaWiki, DokuWiki, Qt and some distributions of Linux. The program is free and released under the GNU GPL license (version 2).
Subversion (since 2010 called Apache Subversion) is a free system of version control. Subversion is designed specifically for replacing of CVS (the most common open version control system). It has all basic functions of CVS (although some of them it performs in another way) and it is free from some of its shortcomings. Subversion is often called «svn», under the name of the client program that is the part of its distribution.
Mercurial is also known as Hg (the symbol of the chemical element mercury). It is a cross-platform distributed version control system, which is designed for effectively work with large repositories of code. Primarily it is the console application.