TeXShop for Mac is a TeX previewer for macOS, written in Cocoa. Since pdf is a native file format on macOS, the app uses 'pdftex' and 'pdflatex' rather than 'tex' and 'latex' to typeset in its default configuration; these programs in the standard TeX Live distribution of TeX produce pdf output instead of dvi output.
TeXShop uses TeX Live, a standard distribution of Tex programs maintained by the TeX Users Group (TUG) for macOS, Windows, Linux, and various other Unix machines. The distribution includes tex, latex, dvips, tex fonts, cyrillic fonts, and virtually all other programs and supporting files commonly used in the TeX world. The most recent version of this distribution is maintained for the Mac by the MacTeX TeXnical Working Group of the TeX Users Group and available under the 'Obtaining' tab. The app is distributed under the GPL public license, and thus free. The latest version of TeXShop for macOS requires Mavericks, El Capitan, or Yosemite.
The standard TeX distribution on the Macintosh is called TeX Live. BasicTeX is a small subset of TeX Live, but anyone serious about TeX should obtain the full distribution. The MacTeX Working Group from the Tex User Group (TUG) constructed an install package which installs TeX Live and everything else needed to run TeX on Mac OS X in one step. This package is free, and uses Apple's standard installer; installation takes four to eight minutes and is automatic. The package installs TeX Live, the complete reference edition of TeX produced in cooperation by TeX User Groups across the world. It also installs Ghostscript and several GUI utilities for TeX including TeX Shop, so it is not necessary to get the front end separately. One of the GUI programs it installs is 'TeX Live Utility,' which can keep TeX Live up to date. Everything is completely configured and ready to use once the installer finishes its job.
Note: Requires 64-bit processor.