They vary considerably in their brightness. In fact, at any one time there are almost certainly several comets visible to astronomers with large telescopes. Occasionally, the comet may be very obvious, with a bright head or coma and a long tail trailing away.
Comets move very slowly across the sky.
If you look up into the night sky and see a brief streak of light, then you have seen a meteor . These objects vary in brightness, from those far too faint to be seen with the naked eye, to the fireballs which occur very occasionally. Fireballs can be so bright that they cast shadows. At certain times of the year, meteor showers happen when many more meteors than normal can be seen. Asteroids are much harder to see than either comets or meteors, and are particularly hard to find unless you have a pair of binoculars or a telescope, a dark sky, and know exactly where to look.