What is MinGW?

MinGW ("Minimalistic GNU for Windows") refers to a set of runtime headers, used in building a compiler system based on the GNU GCC and binutils projects. It compiles and links code to be run on Win32 platforms... providing C, C++ and Fortran compilers plus other related tools. If you see references to "mingw32" instead of "MinGW", they are referring to the same compiler system. The project's name changed from mingw32 to MinGW is to prevent the implication that MinGW will only works on 32 bit systems (as 64 and higher bit machines become more common, MinGW will evolve to work with them). MinGW uses the Microsoft runtime libraries, distributed with the Windows operating system. Unlike other ports of GCC to Windows, the runtime libraries are not distributed using Gnu's General Public License (GPL). You, therefore, do not have to distribute your source code with your programs unless, of course, you use a GPL library in your programs.

http://www.mingw.org

What is MSYS?

MSYS was created out of a long-lived desire to provide the MinGW community a Minimal SYStem, with which a configure script could be executed. 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.

http://www.mingw.org/msys.shtml