A tsunami is, simply put, an enormous wave that destroys anything and everything in its path. But, that is not usually what it looks like. Tsunamis are caused by an underwater disturbance, such as an earthquake, landslide, volcanic eruption, or meteorite. If you are on the coast watching the tides, you will see that the water recedes quite a lot--about 10 feet. Even though people notice this happening, they just dismiss it as weird and nothing to worry about. Then, small ripples begin to swirl further away from the coastline. After that, a larger wave comes upon the coast--the wave may be tall or short, but that doesn't really matter in the long run. Water starts rushing in from the ocean, rapidly flooding the land and creating extremely strong currents. It is very much like what would happen if you let the bathtub overflow for a long time; the bathroom would be completely flooded, while water is still rushing over the side of the bathtub.
Tsunamis always happen on coastlines, and more than often near the Pacific. The Pacific Ocean has dense oceanic plates that slide under continental plates because they are lighter, and this results in an earthquake, the most common generator of a tsunami. A tsunami-causing earthquake could occur very near the coastline, or hundreds of miles away, but both are immensely destructive. An area in the Pacific known as the Ring of Fire (because of the area's high volcanic/earthquake activity) is a very common place for tsunamis to happen; the surrounding countries are most frequently affected--Chile, Mexico, and Japan are some examples.
Preparation and Aftermath.
We have been able to come up with a warning system for tsunamis, but that doesn't mean that we can always be accurate in these predictions. Ever since 1946, we have predicted 20 tsunamis by measuring seismic waves and using tide gauges to measure the water levels. Although we have accurately predicted 20 tsunamis, 15 of them were considered false alarms because they did not make very large impacts.