Cygwin is a popular complete POSIX emulation environment for Windows. Most of its tools can be used with Emacs, and it covers a wide range of ported software. The main shell used by Cygwin is GNU bash, but other shells are also available. Some Cygwin tools may not interoperate well with Emacs or other native Windows tools, due to the total immersion aspect of Cygwin, including its non-native filesystem mapping.
If you choose to use Cygwin, then its tools will probably be all that
you need, but you will need to get image libraries from elsewhere, as
the Cygwin ones are not compatible with non-Cygwin software. In fact,
if Cygwin is on your PATH when you run Emacs, and Emacs does not find
other versions of the image libraries first, then the Cygwin ones can
cause problems. Cygwin developers recommend that you do not put
Cygwin on your system PATH for this reason. Instead you can
make the Cygwin tools available within Emacs by setting
in your init file.