Why is the control of insulin called "negative" feedback? How does it work?

Negative is a slightly confusing term, as it seems to imply that the control is bad, when in reality it is vital to being alive! Negative in this sense is used in a mathematical way, meaning "removing", ie. insulin stops being secreted when it is no longer needed. This is an example of a feedback system, allowing the body to strictly control the quantities of glucose ("sugar") in the blood stream, which helps prevent there being either too much, or too little. Essesntially, the body uses sensors to detect when the amount of blood glucose changes, then sends signals to the pancreas to tell it about this change, which allows insulin release, and lowering of blood sugar. Once this has been achieved, the release must be stopped, or else it would go too far and there wouldn't be enough sugar, which is just as bad. This is why it is "negative" - once at the correct level, the system switches off the signal.