What is essence of computer systems and networks?

computer is a programmable device that can automatically perform a sequence of calculations or other operations on data without human aid. It can store, retrieve, and process data according to internal instructions

A network consists of two or more computers that are linked in order to share resources (such as printers and CDs), exchange files, or allow electronic communications. The computers on a network may be linked through cables, telephone lines, radio waves, satellites, or infrared light beams.