Computer systems are complex machines that allow users to interact with data and programs. They are composed of several components, including an operating system, hardware, and software. The three crossword solutions for the term 'computer system' are UNIX (4 letters), NETWORK (7 letters), and DOS (3 letters). UNIX is an operating system developed in the 1970s that is still widely used today. It is a multi-user, multitasking system that is highly reliable and secure. NETWORK is a system of interconnected computers that allows users to share resources and communicate with each other. It is composed of hardware, software, and protocols that enable communication between computers. DOS is an acronym for Disk Operating System, which is a type of operating system developed by Microsoft in the 1980s. It is a single-user, single-tasking system that is used to manage files and run programs on personal computers.