System software is a computer software which provide a platform for application software. There a four types of system software: Operating systems - like Microsoft Windows, Linux, MacOSX Compilers...
Actually, a system software is any computer software which manages and controls computer hardware so that application software can perform a task. Operating systems, such as Microsoft Windows, Mac OS...
The operating system is one type of system software. The operating system is the software that your computer runs on such as windows xp and windows 7 or mac.