While TeX Live is designed to be cross-platform (running on Unix, macOS, and Windows), MacTeX includes Mac-specific utilities and front-ends (such as TeXShop and BibDesk).
[1] It is also pre-configured to work out-of-the-box with macOS, 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 full MacTeX install package contains three subpackages:[1] A substantially smaller version of MacTeX, BasicTeX, which does not contain Ghostscript or the aforementioned GUI programs, can be used instead along with a TeX editor as well.