What exactly is ketosis?

Can anyone explain exactly what is ketosis? I'm a bit confused!!!!

Ketosis defined

Hi Sam
keton bodies are a byproduct of protein metabolism in the body and can be used for energy. It can lead you to feel tired, weak and irritable and excessive and prolonged ketosis can have very serious and harmful effects.

Nick
Nutritionedge@gmail.com

I think...

I believe Ketosis is when your body goes into starvation/fat burning mode (please feel free to correct me, anyone!). I know that on some diets you eat nothing apart from a few specially designed foods and your body is in ketosis because it gets used to not receiving much food and concentrates solely on burning what you have on your body already. You often don't have much of an appetite whilst in ketosis either.