I'll try. The vast majority of transmission and distribution systems in the world today are alternating current. Aside from the history lesson, part of the reason that's done is that you can use transformers to transform voltage relatively effectively, efficiently, and cost effectively. One of the things that designers of transmission networks do is that they attempt to use the highest voltage possible, reduce the current, and by doing so, they reduce losses, so it becomes the most efficient way to transmit volumes of electricity.
If you have a very long distance, as Quebec would have had with their early James Bay projects and Manitoba Hydro has done, you have significant losses over these long distances. Therefore, traditionally DC, or direct current, was reserved for these very long, substantial transmission projects. They convert alternating current in a converter station, they transmit it on direct current transmission lines, and they reconvert it at the other end back to AC.
Historically, those converter stations have been very expensive. They're essentially electronics. For the same reason that your phone.... I don't know if it's cheaper, but your television is cheaper than it used to be. Converter stations have become cheaper than they used to be, so the technology has caught up to us to now allow relatively efficient and lower cost conversion to DC.
The advantage of a DC system is that it gives you a gas pedal, like in your car. You can actually control the amount of flow that goes down the transmission line, whereas an AC system just allows the flow to occur. It gives system designers and operators some more control and more flexibility. They can actually force power to leave. They can import power. It's still generally considered economical, but only for relatively long distances.