International Meteor Organization - Glossary of terms
Comets
Comets are small celestial bodies (with a diameter varying from a few kilometres to at most a few tens of kilometres) that revolve around the Sun in long elliptical orbits. Comets consist mainly of ice and dust. When a comet approaches the Sun, part of the ice will evaporate and, because of the resulting pressure, the gas will find its way through cracks and fissures in the thin comet crest and be ejected under the form of "geysers." The evaporated ice of these geysers will feed the coma and the tail of the comet. Together with the evaporated ice, a lost of dust is released. This dust eventually spreads along the entire orbit of the comet, but remains densest in its immediate vicinity. Meteoroid streams usually consist of cometary dust.
Meteors
Dust particles orbiting the Sun and capable of "colliding" with the Earth are called meteoroids. Such a meteoroid has usually the size of a sand grain or a tiny stone. When it enters the atmosphere, with typical velocities of a few tens of kilometres per second - several then thousands of kilometres per hour! - not only the meteoroid but the surrounding air experience enormous friction. This friction causes the air surrounding the meteoroid to give light, in much the same way as an electric current causes the gas in a TL lamp to give light. Meteors typically light up at heights of 90 to 110 kilometres. The resulting light is called a meteor or a shooting star. Usually, the enormous friction causes a meteoroid to disintegrate into the molecules it is composed of: the meteoroid "evaporates" completely. Only the larger and stronger meteoroids may survive traversing the atmosphere. The remainder of the meteoroid that impacts on the Earth is called a meteorite. Leonids are too fragile to produce meteorites, even if they are meter-sized.
Occasionally, meteors are exceptionally bright, brighter that the brightest planets, and sometimes even brighter than to Moon. These meteors are called fireballs.
Meteoroid streams and meteor showers
The collection of particles released by a comet (or comet-like asteroid) is called a meteoroid stream. The meteor display in the sky caused by a meteoroid stream is referred to as a meteor shower, or, in case of extremely high activity, a meteor storm.
Meteoroid streams and their associated meteor showers are named either after the comet from the particles originate, or, as is the case for the Leonids, after the constellation in which its radiant is located.
The Leonids are not the only meteor shower we can see. In fact, there are dozens of other meteor showers, but most of them never produce more than a few meteors per hour. Two notable exceptions are the Perseids, active around August 12, and the Geminids, active around December 14. Every year, both showers produce several tens of meteors per hour at their respective peak times.
Finally, we must mention that the Solar System contains a lot of dust particles that do not belong to any particular meteoroid stream. These particles cause so-called sporadic meteors, which may appear any time.
Radiant
Meteoroids of the same stream orbit the Sun along a common orbit (roughly the orbit of the comet from which they originate). When the Earth crosses a meteoroid stream, our planet is "hit" by a "bombardment" of dust particles which all come from the same direction. The perspective, however, leaves us the impression that the meteor trajectories in the sky, when prolongated backward, originate from a single point, just like the tracks of a long, straight railroad. This point is called the radiant of the meteor shower. Most meteor streams and showers are named after the constellation in which this radiant is located. Even though the backward prolongations of all meteors of the same shower intersect the radiant, the meteors themselves can appear anywhere in the sky. Hence, there is no need to look in the direction of the radiant to observe a meteor shower.
To understand better what happens during a meteor shower, picture the stream orbit as a "race track" along which all meteoroids race at the same speed. Picture the Earth, with yourself as an observer on the Earth, as an "unexpected" obstacle on this racetrack with which meteoroids may collide, producing meteors in the process. If you look in the direction of the radiant, you will only see short meteors, caused by meteoroids colliding with the Earth's atmosphere while "running" almost straight toward you. If you look at 90 degrees from the radiant, you will see long meteors, caused by meteoroids colliding with the Earth's atmosphere just when they were about to overtake you. If you watch even further away from the radiant, you will again see shorter meteors, caused by meteoroids colliding with the Earth's atmosphere after they had passed you, and thus moving away from you.