A tornado is a funnel shaped, violently fast moving, vortex of air that typically occurs in thunderstorms. It's formed when a cool air mass and a warm air mass meet and cause instability in the atmosphere.
How Does a Tornado Form?
A tornado typically starts with a thunderstorm in a low pressure system (A low pressure system is a mass of warm, moist air that typically brings stormy weather with high winds). Warm, moist air from the Gulf of Mexico and cold, dry air from Canada intersect and create instability in the atmosphere. The two air masses are at two different altitudes and blowing at two different speeds, creating a rotating column of air. The column of air spins faster, and if this air column gets caught in an updraft (an updraft is an upward current or draft of air), the updraft makes the column spin faster, creating a funnel cloud (A funnel cloud is a rotating funnel-shaped cloud forming the core of a tornado). The precipitation (In this case, rain and hail.) in the storm cause the cloud to touch down, forming a tornado.
A tornado Just south of Campo, Colorado on May 31, 2010
On March 18, 1925, the Tri-state tornado touched down in Missouri, Illinois, and Indiana. It traveled along a 219 mile path of destruction, killing 689 people, injuring 2,000, and leaving more than 10,000 people homeless. The price of the damage was estimated at $18 million in 1925 US dollars.
Comparatively, On May 22, 2011, A tornado touched down in Joplin, Missouri. It was on the ground for a little over 22 miles with winds over 200 mph, killing 158 people, and injuring about 1,000. The price of damage was $2.8 billion US dollars.
The main reason for less deaths and injuries in more recent tornadoes is that in 1925 and before, people didn't have as much - or any - technology to prepare for tornadoes as they did in 2011 and as they do now, so there were much more deaths and injuries. While we now have technology that can predict tornadoes weeks in advance, they didn't have any form of warning besides the recent weather to tell if a tornado was coming.
Impact on Human Life
Humans have had to make adaptations to help us survive in harsh weather like tornadoes. To help warn people about tornados, scientists have made radars (more specifically, the Doppler Radar, a radar that uses the Doppler effect to determine the location and velocity of a storm, clouds, precipitation, etc.) to track and predict when a tornado or weather that would cause or indicate a tornado near you.
Humans have also made sirens that warn you when a tornado or other harsh weather condition is coming so that you can get to somewhere safe, which has greatly helped reduce deaths and injuries.
A video of the tornado in Joplin, Missouri, 2011.