MacTeX is a redistribution of TeX Live, a typesetting environment. While TeX Live is designed to be cross-platform (running on Unix, Mac OS X, and Windows), MacTeX includes Mac-specific utilities and front-ends. It is also pre-configured to work out-of-the-box with Mac OS X as it provides sensible defaults for configuration options that, in TeX Live, are left up to the user to allow for its cross-platform compatibility.
MacTeX is packaged and distributed by the MacTeX TeXnical working group, a subgroup of TeX Users Group (TUG). TeX Live is distributed by the TUG, making MacTeX less a fork of TeX Live than a customised repackaging.
The MacTeX-2013 install package contains six subpackages: