In popular culture, tsunami and tidal wave are often used interchangeably, but they have two very different meanings. A tsunami is a very powerful wave originating from the ocean floor, where earthquakes or movement of tectonic plates causes the ocean water to move on an enormous level. A tsunami's wave comes from the water being suddenly pulled into a hollow, causing it to shift its volume suddenly; the tsunami's waves are long and low out on the ocean, but become amplified when they hit the shore. Tsunamis are very destructive, and can wipe out entire coastline cities with the force of water crashing forward and then pulling back.
A tidal wave, on the other hand, is caused by tidal forces of the currents, weather, and the Moon, which is the main cause of all tidal activity. The "wave" is the natural movement of the water, affected by the gravity of the Moon and so rising and lowering depending on time of day. Most tidal waves never reach inhabited areas, because they can be measured and so construction does not take place below their water line. Tidal waves can cause problems, but not on the level caused by tsunamis, and in fact tidal waves are predictable and usually cause no damage at all.