The tsunami is a result of the volcanic and earthquake activity on the plate boundaries in the ocean. It forms when there's a big disturbance in the oceanic crust, and from all the force and stress released, the water is becoming very turbulent.
At first, the waters of the ocean/sea are dragged towards the crack where the volcanic or earthquake activity appeared, and after that, under enormous pressure the water is pushed out again. The water now has very big speed and very big power. As it nears the shore it creates huge waves, and once these waves reach the land they destroy most things in front of them.
