In Summary

Thus, the three key features of "modern" computers are that they are:

• digital
  Modern computers are digital, because they mimic the operations of arithmetic by manipulating devices that represent the digits of numbers.
  
• binary
  Modern computers are binary, because the kind of arithmetic they mimic is not that of our favored decimal (base-10) numbers but, rather, arithmetic operations upon binary (base-2) numbers.
  
• electronic
  Modern computers mechanize arithmetic. In other words, they are machines that do arithmetic, in the sense that they are built to mimic the rules of arithmetic: that is, given some input representing an arithmetic instruction, the machine is built so that, blindly and automatically, its switches flip to the states representing the appropriate result.