Cygwin is a POSIX-compatible programming and runtime environment that runs natively on Microsoft Windows. Under Cygwin, source code designed for Unix-like operating systems may be compiled and run natively with minimal modification. From cygwin.com: Cygwin is a large collection of GNU and Open Source tools which provide functionality similar to a Linux distribution on Windows. Cygwin also provides substantial POSIX API functionality.
Cygwin is available to all users.
Cygwin is only available for Windows.