The joy of electronics questions like this is one of the reasons I love the Pi. Unfortunately wall chargers pretty much everywhere in the world work on AC current, which they turn into DC current that powers the Pi. One letter difference how had can it be? Well, alternating vs. direct as it turns out matters somewhat.
In alternating current, the electrons in the wire bump back and forth, back and forth in the same position. Each electron knocking its neighbour and vice-versa is what carries the power. The fact the electronics essentially never move is what makes it great for transmission over long distances, like from the power plant to your house. However, because the electrons go back and forth, trying to power a car with an AC engine would be like watching it go forwards and backwards repeatedly. (Not to say that you can't power a car with an AC engine, but I'm using it as a metaphor here.)
In direct current, the electrons travel the full distance. This makes the car with that same motor move in one direction only. If we move the car metaphor, where powering of an AC motor is actually trivial, to most consumer electronics, powering them with AC is non-trivial. Hence DC. Why not use DC for everything? The fact that all the electrons are moving means the experience resistance (think friction) which causes them to heat up (hot wires) which further increases resistance (you slow down on a hot day) which means you have to put more power in. In room temperatures it's a loosing proposition. (In near absolute zero conditions it works amazingly well---that's where the super conductors of fabled sci-fi enter in.)
Anyway, the difference between AC and DC is why using the wall wort from DC power won't work. They use clever bits of electronics (previously transformers, now switched mode regulators) to make the transition. But there are such things DC to DC convertors. Switched mode regulators do that too. The MoPi board has one, will take any input voltage from around 8 to 25 V (so basically as many AAs as you can find, or any whacky old laptop battery) and make it so that the Pi can use it. It will then also tell you when the battery is about dead. Nifty!