Comets are not bad omens. They are natural phenomena. Comets either originate in the Kuiper Belt, which is a ring of icy debris just beyond Neptune, or the Oort cloud, a huge cloud of debris that lies on the very edge of the solar system. Kuiper Belt comets are short-period comets while Oort cloud comets are long-period comets.
When comets are in the Kuiper Belt or the Oort Cloud, they are simply conglomerations of dust, rock and ice. If something jostles the comet out of its original position, it might start to fall toward the sun and be captured by the sun's gravity.
Comets can be seen in the sky because as they get closer to the sun a glowing envelope of gas surrounds them. Gas and dust can be stretched to form tails that are millions of miles long.
Some comets take millions of years to orbit the sun, while some, like Halley's comet, take only a few decades. Other comets orbit the sun once and are never seen again because their orbits are so eccentric. Once in a while, a comet will get so close to the sun that it will evaporate or plunge into the sun and be consumed.