Its a silicone ball that is inflated in your tummy with saline solution. It's inserted through the mouth - usually under light sedation. Its not very pleasant to have done, but only takes around half and hour - some people stay overnight in hospital. Some people have nausea/sickness for a few days - my sickness passed pretty quickly thankfully - but some can go for months feeling or even being sick.
The idea is that once its inflated it takes up room in your stomach so that you can't eat as much as you used to. Its usually in place for six months ('Spatz' balloons stay in one year) and then has to be removed as it will start to disintergrate because of stomach acids. You need to eat sensibly/healthily with the balloon otherwise you won't lose much weight. Its not a permanet solution and you can regain weight once the balloon's removed, but a lot of people have them to see how they go with restriction, or to lose a little weight to make surgery to safer for them, or to get into good eating habits.