She has Type 1 diabetes. It's genetic and her pancreas no longer produces insulin on it's own. So she has to take insulin for the rest of her life (unless they find a cure) or she'll die basically. So the pump does a couple of things - it constantly gives her a small amount of insulin to keep her level. Then when she eats she has to count carbs and take a set amount of insulin so that her body can absorb the carbs. Currently she does this with shots. She takes one before each meal and one at bedtime. Possibly more if her levels are high. So the pump also means not so many shots.
Most people have Type 2 diabetes which is a whole other ballgame. Type 2 can be controlled by diet and medications. Although some do go on insulin as well.
The other great thing is the pump is more discreet. At lunch at school or at a resturant or whatever she doesn't have to whip a needle out and give herself a shot before eating. Now she just tells the pump how many carbs she's eating and it delivers the insulin through the pod which is attached to her and is filled with insulin. It delivers it through a small cathater which it attaches to her once you put the pod on.
She still has to test her blood glucose as before, this is poking your finger and putting a small drop of blood into a meter which measures it. But less shots at least makes her happy. And it will also help keep her glucose levels more stationary which is also good.
Here's the website for the pump she's got - it has no tubing and is wireless which is extra cool
OmniPod