A computer is a device for manipulating data according to a list of instructions. A list of computer instructions planned to perform some task is known as a program. When programs are contained in storage that may be easily customized by the computer itself, the device is said to have Von Neumann architecture. The ability to execute stored programs that is, programmability makes computers extremely versatile and distinguishes them from calculators. The Church Turing thesis is a mathematical statement of this versatility: Any computer with a certain minimum potential is, in principle, capable of performing the same tasks that any other computer can execute. Therefore, computers with potential and complexity ranging from that of a personal digital assistant to a supercomputer are all able to perform the same computational tasks so long as time and storage capacity are not considerations.
Computers take several physical forms. Early electronic computers were the mass of a large room, consuming as much power as several hundred modern personal computers. Today, computers can be made little enough to fit into a wrist watch and powered from a watch battery.