TortoiseHg is a set of graphical tools and a shell extension (or an interface that provides specific handling for objects in which it is programmed to handle) for the Mercurial source control management tool. It is open-source under GPL.
On Windows, TortoiseHg consists of a shell extension, which provides overlay icons and context menus in your file explorer, and a command line program named thg.exe which can launch the TortoiseHg tools. Binary packages of TortoiseHg for Windows come with Mercurial and a merge tool and are thus completely ready for use "Out of the Box".
On Linux, TortoiseHg consists of a command line thg script and a Nautilus extension which provides overlays and context menus in your file explorer. You must have Mercurial installed separately in order to run TortoiseHg on Linux. TortoiseHg binary packages list Mercurial as a dependency, so it is usually installed for you automatically.
It is designed for Windows XP, Vista, 7 as well as Mac OS X and Gnome. TortoiseHg is primarily written in Python and PyQt. Its Windows shell extension relies on windows command line language rather than specifically Python and PyQt.