Tsunami are a series of waves usually generated by movement of the seafloor. These movements are caused by different typs of geophysical phenomens such as earthquakes, landslides, volcanic eruptions, slumps and meteroits. But 90% of the Tsunamis are initiated with earthquakes.
They all repress a enormus mass of water, which become special waves, the ratio between the the water depth and the length of these waves is very small. They move at a speed equals to the square root of the product of gravity (9,8m/s/s) and the depth of the water. Because they lose energy reciprocally related to their lentgh, they can travel with high speed over long distances without losing much energy. Therefore, the tsunami reachs the coast with a destructive and devasting force.
Here are some examples of tsunamis caused by different geophysical phenomens: