Cygwin is a DLL which provides a Unix emulation environment for Windows. The Cygwin environment provides a complete port of such development utilities as gcc, binutils, gdb, make, etc., as well as a vast number of useful utilities.
Cygwin - is a collection of free software tools originally developed by Cygnus Solutions to allow various versions of Microsoft Windows to act similar to a Unix system. It aims mainly at porting software that runs on POSIX systems (such as Linux, BSD, and Unix systems) to run on Windows with little more than a recompilation. Programs ported with Cygwin work best on Windows NT, Windows 2000, Windows XP, and Windows Server 2003, but some may run acceptably on Windows 95 and Windows 98.