A tsunami is the displacement of a large body of water due to an underwater earthquake, that takes place when opposing pieces of the earths crust slip away from each other, one in an upward direction, and the other in a downward direction.  This happens suddenly and it forces the seawater to be displaced.   The seawater has to go somewhere, and it does.  It moves outward in all directions (as if you had thrown a pebble into the pond and it caused the water to fan out in continuous circles) but the energy that forces the seawater outward, creates an enormous wall of water that travels up to speeds of 700 miles an hour.  This wall of water is gigantic and as it comes in contact with shallow water near the coasts of countries, it is forced even further upward into a massive wave of over 1000 football fields long.  This water just keeps coming, sometimes for hours.  One of the worst characteristics of a Tsunami is that it happens without warning, destroys all structures, and kills every living being in its path.