Eyjafjallajökull is a small glacier just west of the medium sized Mýrdallsjökull which hosts the better known volcano Katla.
Eyjafjallajökull has its own, smaller, caldera which last erupted almost 200 years ago, and seems to have erupted every couple of hundred years ago in the last millennia.
The magma system is part of the fissure running from Heimaey up to Katla and on north, and is very active.
Since the volcano is sub-glacial, any significant eruption will be accompanied by flash melting and shattering of the ice cap, and associated jökulhlaup - the wall of water coming down as the eruption starts, which can be tens of meters high and scour a strip many kilometers wide, with peak flow levels larger than the Mississippi in full flood.
To make things more exciting, the magma system may be tightly enough coupled to Katla, that eruptions pair - Katla is bigger and meaner, and historically erupts a couple of times per century. It last erupted 92 years ago.
A couple of time per 100,000 years Eyjafjallajökull seems to undergo major explosive eruptions.
http://scienceblogs.com/catdynamics/2010/03/eyjafjallajkull.php