4. And how is that different from digital?

Digital signals are not continuous. They are discrete, which means that they send a series of samples of an audio signal's power at precise intervals. Sound does not naturally break down, so a digital system subdivides it into bits, the smallest possible form of information. This is binary code, so everything is broken down into one of two directives, which is typically described as 0 and 1. The benefit of binary code is that by breaking down information to its smallest possible form, it can represent virtually anything with only two elements.

To imagine this visually, it's like plotting points on graph paper. This sounds terrible, but the reality is that the end result of an analog or digital signal is exactly the same after it is processed through an amplifier and played through speakers: You hear a continuous sound wave. There is no objective way to analyze the end result to determine whether the source was digital or analog because we cannot actually hear digital code or analog voltage fluctuations.