The solar system form out of an interstellar cloud containing mostly hydrogen and helium, with trace amounts of the heavier elements in the form of "dust". As the cloud collapsed, the Sun formed at the center. But it was spinning, and the conservation of angular momentum caused substantial material to form in a disk around the Sun. In this disk material was eventually collected into planetesimals: bodies a few km in diameter, composed of primarily rocky material in the hot inner solar system, and mostly icy (with some rocky) material in the outer solar system (due to the varying temperature moving away from the Sun). Some of these bodies survive today as asteroids and comets.

The asteriods are concentrated in the region between Mars and Jupiter. Some are mostly iron which means they must have been part of a larger molten body which differentiated (separated out by density), and was then broken up releasing iron core material. This is probably a result of the influence of orbital perturbations (disturbances) due to Jupiter. We still see some influences from orbital resonances with Jupiter. Some of the planetary moons are very likely captured asteroids. Other asteroids are rocky; closer to the original rocky planetesimals. Some asteroids are on Earth-crossing orbits, and pose a (small) threat of future giant impacts. The largest asteroids are hundreds of km in diameter, but most are smaller (too small to be crushed to a spherical shape by their own gravity). The asteroids are very far from each other (millions of km) on average, but occasionally collide producing fragments. These are a main source of meteors striking the Earth.

The icy planetesimals are also still around in large numbers; we call them comets. There are two main populations. The ones which are still in the plane of the Solar System (left over from planet formation) form a population of objects beyond Neptune; Pluto is the largest known such body (of course it is an accumulation of many planetesimals). This is call the Kuiper Belt (also here). That is the source of the short period comets with orbits near the ecliptic. The giant planets also perturbed the orbits of many comets during the early solar system; these were ejected into a giant cloud of comets surrounding the solar system and extending out 50,000 AU or more: the Oort cloud. These comets are on very elliptical orbits which come down into the outer solar system, but they spend almost all their time very far away. Occasionally one comes in and is perturbed again into a short period orbit going into the inner solar system (like Halley's comet).

When a comet approaches the Sun, its ices begin to vaporize, and gas and dust blow off the nucleus (which is only a few km in diameter). These make a smaller bright cloud around the nucleus called the coma (which might be a million km in diameter) and an extended tail (which might be 100 million km long). The tail has a dust component, which is pushed back by sunlight, and an ion (charged particle) component which is blown straight back away from the Sun by the high speed solar wind. Thus, the tail always points away from the Sun, rather than away from the direction the comet is headed. The comet nucleus is mostly ice, with rock mixed in, and covered with an organic "tar" which makes it quite dark (the bright comet we see is sunlight reflecting off ejected dust). Comets are not structurally strong, we have seen many instances where they will break apart under stress. As the Sun evaporates the ice away, the comet will eventually be reduced to a stream of rocky debris. If we encounter such a stream, it makes a meteor shower.

Meteors are rocky chunks which encounter the Earth's atmosphere. They are typically quite small (pea-sized or less), and burn up in the upper atmosphere due to friction, leaving a very temporary glowing trail (often called a "shooting star" or "falling star"). If the object is bigger (and especially if it is a metallic body), it may reach the ground, becoming a meteorite. Since the typical entry velocity is tens of km/s, there is a lot of energy of motion. Objects the size of boulders  will hit with the impact of a nuclear bomb, leaving a crater. Rocky meteors may break up before reaching the ground, but a strong pressure shock can still have unpleasant effects (for example, the Tunguska event in 1908) . Much bigger objects pose a serious threat; they pass through the atmosphere (or even the ocean) without much trouble. The impact vaporizes the object and an equivalent amount of our crust, causing tremendous shock waves, heating, debris injected into the atmosphere (or orbiting or escaping the Earth), and climatic effects that can be severe and longlasting. It is thought that such an giant impact played a role in the extinction of the dinosaurs (but they only occur every 100 million years or so). If the object were 10s of km in diameter, it would replace our whole atmosphere with rock vapor -- good-bye!