Software are the step-by-step instructions that tell the computer what to do. In general, software is divided into applications software and systems software. Applications software, which may be customized or packaged, performs useful work on general purpose tasks. Packaged software includes word processing, spreadsheet, database manager, graphics, and communications programs. Systems software which includes operating systems, enables the applications software to interact with the computer. Principal microcomputer operating systems or operating environments are DOS, Windows, OS/2, FreeBSD, and UNIX.