Can someone explain how ketosis works?

First, I must please exhort everyone NOT TO TURN THIS INTO A GREAT DEBATE! I am aware that whether or not ketosis is beneficial is a big topic of debate among those who like and dislike diets such as the Atkins Diet. That has nothing to do with my question.

I would just like somebody to cogently and coherently explain what the process of ketosis is, and how it works. I’ve recently been diagnosed with gestational diabetes, and one of the things I do is check my urine for ketones every morning. If my ketone level is too high, they have me eat more carbohydrates. Now, I’ve done a lot of snooping around on the Web, and all I’ve found are either 1) extremely technical explanations involving medical terms that I don’t understand – but I would understand them if a little more basic explanation was provided!, 2) overly simplified explanations designed for fifth-graders, or 3) huge debates about the Atkins diet.

All my diabetes educator told me was that ketosis happens when your body breaks down fat. OK, but if that’s true, it would happen every time I lose weight, right? And I know for a fact that having way too many ketones in your bloodstream is a bad thing, and I know that losing weight isn’t necessarily bad and doesn’t necessarily send you into ketosis (er… or does it?), so what’s the scoop?

I just need a little explanation. And please, again, let’s not turn this into a debate.