Understanding asynchronous operations

It's a word you've probably heard before, but may not understand exactly what it means or why we care. Asynchronous operations are operations that do not block while completing. What does that mean, exactly? Well, synchronous operations (the vast majority of commands in a program) get executed one at a time, in order. Consider this code:

mixBatter()
bakeCake()
frostCake()

Each of these functions is invoked in order, and not until the previous function returns. bakeCake is only invoked after mixBatter returns, and frostCake is invoked only after bakeCake returns. If bakeCake takes 40 minutes to return, well, frostCake will have to wait 40 minutes before it can run. We could say that bakeCake blocks execution until it is finished.

This makes the code very easy to reason about, and most of the time it's the behavior that we want. However, there are times where we might wish to start an operation but not wait for it to finish before doing more work. Most often...