A hurricane is a large storm system that forms in the atmosphere over warm ocean water. A tsunami is large set of waves triggered by some sort of vertical movement of rocks under water (earthquake, large landslide, volcanic explosion). Both of them can be very severe when it comes to impacting humans, but overall hurricanes have probably caused more damage and deaths.

We have seen in the last month (today is January 11th, 2005) that a large tsunami can kill more than 150,000 people and do untold damage, but hurricanes are a lot more common and have killed about 600,000 people in the last 50 years (most of them in low-lying Bangladesh).

Hurricanes have very strong winds spinning around the center of the storm. These winds push up a mound of water called a "storm surge". In the most intense hurricanes, this storm surge can be 7-8 m high (~25 ft). It does not arrive nearly as quickly as tsunami waves, but it can stay for many hours and along with >100 mile/hr winds and lots of rain, it can cause a lot of damage.

However, the best defense against both hurricanes and tsunamis is early warning systems (easier for hurricanes) and leaving coastal areas when instructed by authorities (or in the case of a tsunami - as soon as the earth stops shaking, leave the beach!).

A hurricane is a storm in the atmosphere; a tsunami is a huge tidal wave in the ocean, caused by a large under thrusting earthquake.

They are entirely different phenomena. Both have the potential to be very damaging. The thing about a tsunami that makes it worse is that, as happened in Indonesia, there is little advance warning without the proper detectors installed at sea, so the people could not escape. You usually have enough time to get out of the way of a hurricane, and get to shelter. The force of so much water is greater than that of a hurricane, but hurricanes also last longer and so can cause lots of damage also. I have never heard of a hundred thousand people dying from one hurricane, though. But - don't worry. We have a good tsunami warning system in California, in case there is any great earthquake in Alaska that causes a tsunami in our side of the Pacific Ocean, so we can get out of the way. They did not have this in Indonesia and India - and you know all about the damage, from the news reports.

They are not the same thing at all.

A hurricane is essentially a gigantic thunderstorm that swirls in huge regions of cloud, with colossal winds, very low atmospheric pressure (for sea level), and torrential rain.

A tsunami is a pressure wave generated by a change in the shape of the sea-floor, particularly by a change in the depth of the sea-floor (itself by an earthquake).

You can make a little tiny tsunami in a bathtub. When you insert your body into the bathwater, you generate a pressure wave. If you go deeper, so that more of you is submerged, you generate a pressure wave.

Obviously, if the area being submerged is the size of the state of California and is moving several meters (the recent earthquake off Sumatra), the displacement of water is going to be huge, and the tsunami will be similarly huge. Hurricanes, by the nature of air flow, have to be big. Every few years, a typhoon (another word for hurricane, just one that hits Asia) slams into Bangladesh, a lowland country. The entire country is flooded, and it kills well over a hundred thousand people. The recent tsunami in the Indian Ocean killed a similar number.