The Asteroid Belt is a group of thousands of rocks that orbit in between Jupiter and Mars. Some of these asteroids have orbits that actually DO come inside of Mars' orbit and cross the path of the earth. These are called Earth Crossing Asteroids, or ECAs. This is what has scientists woried about a collision with earth because we only know the positions of about 200 of these, and we still know that there are many more out there. But don't worry, we would have years to prepare for a possible collision, unlike the 18 days that they get in Armagedon. By that time, the asteroid would be visible to the naked eye in broad daylight.
In 1866, Daniel Kirkwood realized that gaps in the distributions of asteroids around the Sun were simple fractions of Jupiter's orbital period. For example, a big gap in the asteroid belt is located about three and a quarter times the Earth's distance to the Sun (that distance is sometimes called an "astronomical unit, or AU for short). An asteroid in this orbit would be in a 1:2 ratio of Jupiter's orbit (Jupiter takes 12 years or so to revolve around the Sun, and an asteroid 3.28 AU from the Sun orbits in about 6 years). Many other such gaps exist as well. Honoring the man that figured it out, these gaps are known generically as "Kirkwood gaps" (or sometimes "Kirkwood resonance").
