This is when one computer watches another computer. But it’s more than that. A lot more. First, the watching-computer can check the computer-being-watched in a lot of different ways. Some ways are fast and simple. Others take longer and are harder to set up but get a lot more information. There are over a dozen ways, and I even wrote a whole book about it. But the point is each way of computer-watching gets different information to help in its own way. You need to understand all the different kinds of computer-watching, so you have lots of choices and get all the information you need to keep your computers running in a way that makes them happy and makes the people who use them happy, too.