"Msys is just a native compiled bash plus some other native compiled gnu tools. . . . The point is there is a minimal posix-style command line environment that should just be enough to run configure scripts. Unlike cygwin which gives you a full linux-style command line and runtime environment. . . . It's a minimal cygwin. . . . The main issue is that the configure result is suitable for mingw gcc to generate native compiled binaries."
http://www.mingw.org/msys.shtml
What is MSYS? from the site
MSYS or Minimal SYStem is a POSIX and Bourne shell environment use with MinGW. It provides a hand picked set of tools to allow a typical configuration script with Bourne syntax to execute. This allows most of the GNU packages to create a Makefile just from executing the typical configure script which can then be used to build the package using the native MinGW version of GCC.
The POSIX layer used by MSYS is a fork of the 1.3.3 version of Cygwin. Cygwin is a full POSIX layer and UNIX-like environment for Win32 providing both server and client utilites.
For more MSYS-specific information, please see the MSYS section of this website: http://www.mingw.org/msys.shtml .
See also