An house-plant needs just the right amount of light, water, food, soil pH etc... too much or too little of any of these can lead to the leaves dropping off or worse.
In much the same way: our bodies have systems which require an internal balance in order to provide the ideal environment for our cells. There are systems which (when working normally) manage this balance or homeostasis -- for example there is a feedback mechanism which uses insulin to lower the BG level and Glucagon to raise BGs... so that the normal non-diabetic person always has BGs in the normal range
The fluid balance and pH is also controlled by similar mechanisms.
Ketosis (raised level of Ketones) is our normal physical fasting state between meals and overnight when the body is using Fatty Acids as the major energy source. Many folks rarely see this these days as the Western diet keeps them constantly topped up with refined Carbohydrates. On a low-carb diet you may test positive for Ketones BUT so long as your BG is well controlled there is no cause for alarm.
Ketoacidosis (Diabetic KetoAcidosis - DKA) is a dangerously high level of Ketones that changes the pH (Acid:Base) balance of the body fluids. DKA is most commonly (but not exclusively) seen in an untreated Type 1 Diabetic where there is a lack of insulin -- pump failed, insulin lost potency, insulin not given etc... -- and is usually accompanied by dangerously high BG levels.
You would need to watch for raising BGs and positive test for Ketones... in which case you need to stay well hydrated, and check that your insulin is working... if in doubt seek medical help.
We are actually using fatty acids much, if not all, of the time as an energy source but after a meal including carbs we tend to use the carbs first -- some will say that is because it is our preferred fuel, but I might argue that it is because high BGs are toxic and we need to get rid of it.
So despite what you may read online (and there is much misinformation on this topic), burning Fat as an energy source is NOT a last ditch effort by the body to keep us from starvation, and ketosis IS a normal state of affairs.
In fact I recently read that infants feeding on Mother's milk are in ketosis much of the time.
Frank gave an excellent explanation, I doubt I can add anything. Many people confuse being on a Ketogenic diet and making ketones while bgs are low and DKA with very high bgs. On a low carb/high fat diet your body doesn't need to produce a lot of insulin to push glucose into cells. You are actually burning the fat as energy. That is why it is good for weight loss. All the glucose we need comes from the veggies and protein we eat. As long as we keep our bgs in a healthy range, we stay very healthy. This diet allows me to keep my bgs very close to a normal, non D.