Eyjafjallajökull is actually a glacier in Iceland which covers a volcano (1,666 metres or 5,466 ft in height) which has erupted relatively frequently since the Ice Age. The volcano eruption in 14 April (see pictures) caused massive disruption to air traffic across Northern Europe. From Wikipedia:
"On 14 April 2010 Eyjafjallajökull resumed erupting after a brief pause, this time from the top crater in the centre of the glacier, causing meltwater floods (also known as jökulhlaup) to rush down the nearby rivers, and requiring 800 people to be evacuated. This eruption was explosive in nature and it threw volcanic ash several kilometres up in the atmosphere which led to travel disruptions in northwest Europe on the 15th and 16th of April 2010 including the closure of airspace over most of Northern Europe."
The above picture (click to enlarge) will explain why the volcanic ash is dangerous for Flights.
Europe flights grounded for third day in a row causing disruptions to thousands of passengers. Airlines worldwide are losing at least $200 million a day in revenue because of this.
"The dust cloud from Iceland’s Eyjafjallajökull volcano may affect 6 million passengers if the disruption extends to April 18, and the revenue loss may reach $1 billion, according to the Centre for Asia Pacific Aviation, a Sydney-based consulting firm."Source of Images:
1) Volcano Eruption - by Flickr user Sveinn71
2) Hazards to aviation from volcanic ash cloud - by The Christian Science Monitor
3) Ash cloud impact in Europ - BBC News