Darwin is the UNIX technology-based foundation of Mac OS X. Darwin integrates several technologies. Among the most important are 4.4BSD-based operating-system services (built on the Mach 3.0 microkernel), the I/O Kit, networking facilities, and support for multiple integrated file systems. Developers can use Darwin to port UNIX/Linux applications and create kernel extensions.
In addition to being part of Mac OS X, Darwin is a standalone, BSD-based operating system. (BSD, short for Berkeley Software Distribution, is a family of UNIX variants descended from Berkeley’s version of UNIX.)Darwin is also occasionally used to refer to the Darwin Streaming Server, also known as the QuickTime Streaming Server (QTSS).