The asteroid belt has an estimated total combined mass of less than 1 tenth of the Earths moon. Jupiter also has a profound effect on the asteroid belt. Since Jupiter has a semimajor axis of 5.2 AU (I AU is the distance from the Sun to the Earth) it has an orbital period of 11.86 years. Since the asteroids are not all at the same distance from the sun, some of them have an orbital period of one half of Jupiter. This puts that asteroid in a 2:1 orbital resonance with Jupiter. The result of this resonance is gaps called Kirkwoods gaps. So here is the rub; why did not these asteroids form a planet? The reason is the gravitational force of Jupiter. It perturbs the asteroids giving them random velocities relative to each other. Another effect of both Jupiter and the Sun on the asteroid belt is a group of asteroids that both precede and follow Jupiter in its orbit by 60 degrees. These asteroids are known as the Trojans.
Yepp! Also known as Lagrange points L4 and L5, which are the two stable ones.
Here is a good link on the Lagrange points. And here is the math (PDF!!): http://www.physics.montana.edu/faculty/cornish/lagrange.pdf
However, I'm sure RA knew all this stuff already ;)