One must distinguish between two types of calculators: analogue and digital working. The word "digital" means, that numbers are used for calculation. The statement 6 + 3 = 9 is a digital invoice. The digits only indicate a number, but the type of units, whether it is chairs, stones or kilometers, cannot be seen from the figures. A simple digital calculator is the adding machine in the office.
"Analogue" working automatic calculators always refer to a basic unit. They also determine the size of an existing quantity, but they find the result by measuring, not by counting. These results are displayed on a scale or dial, or a pen draws a line on a strip of paper, which represents the result.
Simple analogue computers are thermometers, watches, Waagen, and in the car it's the fuel gauge and the speedometer. The speedometer converts the rotations of the wheel axle into the numerical value of the speed in kilometers per hour;the axis rotation is the analogy or physical quantity of indicated speed. Electronic analog computers, however, handle much more complicated tasks. They are used for navigation, for flight path calculation for missiles and much more. Electronic digital computers are referred to as data processing systems. You can follow instructions, that are given to them, perform a large series of computational and logical operations. "Logical" operations are sorting, Choose, Compare, Distinguishing and deciding when there are different sizes.