Apple lauched a USB Power Adapter takeback program last year to help costumers replace their counterfeit chargers with Apple chargers, Apple did this due to a few deaths that occurred because of these fake chargers.
You can buy an iPad charger for $19 at Apple Retail and Online stores, while the counterfeit chargers are only a couple of bucks on eBay. Ken Shirriff has written a post on his blog about how dangerous it is to use counterfeit chargers.
At first glance both charger look very similar, except for a small difference in text, the original charger says “Designed by Apple in California. The counterfeit chargers deliver power at 5.9W while the original iPad chargers use 10W, the counterfeit chargers are poorly made, and have major safety issues.
The original charger has more insulation, there is a reddish insulation tape in the lower center of the board which is a safety feature in a genuine charger, the high-voltage half is wrapped in insulating tape, some components are in shrink tubing, plastic insulators are between some of the components, and some of the wires even have extra insulation, as opposed to the counterfeit which has very little insulation. The Apple board is full of tiny surface-mounted components covering most of the space, while the fake one has very few components.
The original Apple charger is of course made with better quality and safety in mind. The counterfeit charger has components that are visibly crooked, crooked components wont affect the circuit electrically but it just shows how these chargers are made without care for quality.
You can also check out his iPhone charger teardown here