Multi-room audio: What is it and how do you get it?

In the past 20 years or so, multi-room music has gone from being a pursuit purely for the pricey custom install market to one that is accessible to pretty much anyone in the market for a wireless speaker. Of course, you can spend several hundreds, and even thousands, if you wish, but you can also quite easily manage a two-room setup for under $100/£100.