A computer is a machine that operates data according to a set of instructions. Although mechanical examples of computers have existed through a lot of evidence human history, the earliest electronic computers were developed in the mid-20th century. Computers are available in different shapes, sizes and weights, due to these different shapes and sizes they perform different sorts of jobs from one another. They can also be classified in different. A personal computer (PC) is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end user, with no intervening computer operator. This is in contrast to the batch processing or time-sharing models which allowed large expensive mainframe systems to be used by many people, usually at the same time, or large data processing systems which required a full-time staff to operate efficiently. A PC may be used at home, or may be found in an office. Personal computers can be connected to a local area network (LAN) either by a cable or with out cable. Proceeding to the wide spread of PCs a computer that could fit on a desk was considered remarkably small. Today the phrase usually indicates a particular style of computer case. Desktop computers come in a variety of styles ranging from large vertical tower cases to small form factor models that can be tucked behind an LCD monitor.
