The program in memory
Early computers such as ENIAC were programmed by plugging in cables and setting switches. Von Neumann described a computer whose program is stored in memory just like the data. Such a computer could switch to a new task in moments instead of days. A processor, memory, and input and output remain the basis of every computer with a microprocessor today.
A genius in many fields
As a child he could reportedly divide eight-digit numbers in his head. He laid the foundations of game theory, used today in both economics and artificial intelligence, worked on the Manhattan Project, and with colleagues devised the Monte Carlo method, which simulates complex phenomena using random numbers.
Machines that copy themselves
Late in life he studied whether a machine could produce a copy of itself. His cellular automata are forerunners of computer simulations of life and of some ideas in artificial intelligence.