A comet is a single very bright star, fired high often at quite an angle to the vertical.
Rows of these fired simultaneously or sequentially form wonderful linking patterns. They are a great surprise effect, and excellent punctuation for choreography. Of course firing these demands electric firing systems, as accurate hand firing is not really practical.
They have other uses as well, for instance to simulate rocket flights at air shows, and using these as tails on star shells to provide added interest to the star shell flight.
Comets come in a range of types, white ones which are excellent to use during the daytime as well as night, ones that leave trials of glittering golden sparks, various colours, and finally a variety which explode at the peak.
These explosive ones are more properly called crossettes, and a usually a smaller variety.