First, here's how to pronounce the volcano in Iceland, “Eyjafjallajökull.” Amaze your friends.
Now here's why the planes are grounded.
The intake of an airplane engine (far left) brings air into the combustion chamber. The particles are small enough that they have no problem getting in past the intake and combustion stages (unlike birds). When the particles reach the combustion stage, however, they get hot enough to soften or melt and instead of going through the engine, they stick on the turbine blades. Once enough of the particles have accumulated, they block the flow of air through the engine, the engine stops generating thrust and the aircraft plunges. This has been documented in about 20 civil aviation craft, in which the plane was essentially a glider for 5-10 minutes. During that time of freefall, the engine usually cools, the particles agglomerates get pushed out and the engines can be restarted. But that's an awfully uncertain way to unclog an engine. [More, including the radar problem]It could last for months, too. This could be a serious economic headache as well, methinks.