Cygwin is a tool that allows you have Linux environment on Windows systems.
It consists of a DLL (cygwin1.dll), which acts as an emulation layer providing substantial POSIX (Portable Operating System Interface) system call functionality, and a collection of tools, which provide a Linux look and feel.
The Cygwin DLL works with all x86 versions of Windows since Windows 95.
It consists of two parts:
· A DLL (cygwin1.dll) which acts as a Linux emulation layer providing substantial Linux API functionality.
· A collection of tools, which provide Linux look and feel.
Cygwin is not a way to run native linux apps on Windows. You have to rebuild your application from source if you want to get it running on Windows.
Cygwin is not a way to magically make native Windows apps aware of UNIX functionality, like signals, ptys, etc. Again, you need to build your apps from source if you want to take advantage of Cygwin functionality.
Note also that, by default, setup.exe does not install everything.
Only the base cygwin distribution is installed by default. When running setup.exe, clicking on categories and packages in the package installation screen will provide you with the ability to control what is installed or updated.
For instance, clicking on the "Default" field next to the "All" category will provide you with the opportunity to install every Cygwin package.
Cygwin can be expected to run on all modern 32 bit versions of Windows, except Windows CE.
Note:
Be advised that this will download and install hundreds of megabytes to your computer. The best plan is probably to click on individual categories and install either entire categories or packages from the categories themselves.

What's New in This Release:
· mkpasswd and mkgroup now try to print an entry for the TrustedInstaller account existing since Windows Vista/Server 2008.
· Terminal typeahead when switching from canonical to non-canonical mode is now properly flushed.
Bug fixes:
· getpass(3) now reads from and writes to/dev/tty in the first place, and only falls back to stdin and stderr if opening /dev/tty fails.
· Fix a bug in execve, which results in wrongly handling arbitrary Cygwin executables as native, non-Cygwin executables.
· Fix a bug in the new /dev code which disallowed to create files under /dev, even if /dev is backed by an existing /dev directory on disk. This could result in misbehaving postinstall scripts.
· (Hopefully) fix a bug in /etc/passwd code which could result in reading unrelated home path entries when accessing the "unknown user account" entry (account name "????????").