What is the difference between an asteroid and a comet?
Asteroids are mostly rock and metal and stay inert, while comets contain a lot of ice and develop a glowing head and tails when they come close to the Sun. Comets also generally formed farther from the Sun.
The classic distinction comes down to ice. Asteroids are largely rock and metal and mostly formed in the warmer inner Solar System, roughly where the asteroid belt lies today. Comets formed farther out, beyond the "snow line" where water and other substances freeze, and are often described as dirty snowballs or icy dirtballs: mixtures of ice, dust and rocky grains.
The difference shows when a comet approaches the Sun. Sunlight warms the surface and ices turn straight into gas, dragging dust with them. This builds a fuzzy cloud called a coma around the solid nucleus, and the solar wind and sunlight pressure push material into tails that can stretch for millions of kilometres. An asteroid, lacking much exposed ice, simply looks like a point of light in a telescope. Comet orbits also tend to be more elongated, and many come from the distant Kuiper belt beyond Neptune or the even more remote Oort cloud.
Nature, though, does not respect neat boxes. Some asteroids in the main belt have been seen to release dust and behave like weak comets; they are called active asteroids or main-belt comets. Some comets that have lost their ice may look like asteroids. Carbon-rich asteroids such as Ryugu and Bennu contain minerals that formed in water, showing that ice once played a part in their history.
Both kinds of body matter for meteor watchers. Most meteor showers, such as the Perseids and Orionids, come from dust shed by comets, while the Geminids are linked to the asteroid-like object 3200 Phaethon. Meteorites, in contrast, overwhelmingly come from asteroids, because cometary dust is too fragile to survive the plunge through the atmosphere.