The geological cause of the earthquake is plain: Indonesia has long been plagued by earthquakes and the volcanoes that usually attend such massive fissures in the earth. Giant plates bearing India and Australia have been moving north for millenniums, colliding with the Eurasian landmass near Indonesia. As a result of the immense pressure that was built up, the southern plate is dug like a bulldozer blade under the northern plate.
When that happened, pressure that had accumulated for years or decades was released in an instant. As the earth convulsed, the ocean floor probably fell rapidly in some places and rose elsewhere along a fissure hundreds of miles in length. Areas that collapsed saw tons of water plunge in, causing what is known as a depression wave. Elsewhere, the ocean floor reared up, causing water to be displaced -- an elevation wave. It is likely that both effects fed the tsunamis.
Think of a kid's plastic swimming pool and sliding your hand underneath -- pushing up on the plastic. It would cause a wave to spread throughout the pool. That's what happens on the ocean floor after an earthquake.
This gives you and good idea of who was hit and how bad.
This shows the concentration of earthquakes around the world. You can see the outlines of the plates and if you look up in the Indian you can see exaclty where it was.
And last a map of the plates
Next week we'll learn about why the sky is blue kiddies