How do monarchies of today differ from those that existed centuries ago?

One difference is the change in the role of the royal family. Years ago a king or queen was an absolute monarch, meaning that they were the source of all power in the country, and they had complete control over their subjects. Here in the UK we have a constitutional monarchy, so the Queen has very little personal power, it is all channeled through the government. She certainly couldn't have anyone's head chopped off!