Cygwin is a software interface that has the look and feel of a Unix environment, but is run within the Windows operating system. Cygwin allows for integration between Windows applications and data and the Unix-like applications and data. It also provides a command line interface for Windows. It was originally developed by Cygnus Solutions in 1995 and later acquired by Red Hat, which continues to update and maintain it today, along with the help of NetApp.
Cygwin is designed with two main parts. One part is a dynamic-link library (DLL), which serves as the API compatibility layer. This DLL provides much of the POSIX API functionality. The second part is a large set of applications and tools that have a look and feel similar to Unix. These applications and tools include file and system utilities, remote login capabilities, and file compression and archiving. They also include support for programming languages like C++, Fortran, Perl, Python, and Ruby, as well as server and client support. Users have ported other Linux and Unix applications to Cygwin, like Apache, GNOME, and X Window System.
Red Hat has kept Cygwin as an open source application, free for public use. Commercial licenses are also available for purchase to users who want to redistribute applications that use the Cygwin library.