Comets have three distinct parts: a nucleus, a coma, and a tail. The solid core is called the nucleus, which develops a coma with one or more tails when a comet sweeps close to the Sun. The coma is the dusty, fuzzy cloud around the nucleus of a comet, and the tail extends from the comet and points away from the Sun.

Comets are basically dusty snowballs that orbit the Sun. They are composed mostly of frozen methane, ammonia and water.

The arrival and appearance of a comet in the inner solar system, where it may be relatively bright for a few weeks or months, is termed an apparition.

The year after the Great Comet of 1618, Gotthard Arthusius published a pamphlet stating that it was a sign that the Day of Judgment was near. He listed ten pages of comet-related disasters, including "earthquakes, floods, changes in river courses, hail storms, hot and dry weather, poor harvests, epidemics, war and treason and high prices". By 1700 most scholars concluded that such events occurred whether a comet was seen or not.

Until the Renaissance, the philosophical consensus on the nature of comets, promoted by Aristotle, was that they were disturbances in Earth's atmosphere. This idea was disproved in 1577 by Tycho Brahe, who used parallax measurements to show that comets must lie beyond the Moon.

As a comet nears the Sun, it warms and begins to release gases, a process called outgassing that produces a visible atmosphere or coma and sometimes also a tail.

Halley's Comet is a short-period comet visible from Earth every 75-76 years. It is the only known short-period comet that is regularly visible to the naked eye from Earth, and the only naked-eye comet that might appear twice in a human lifetime.

Many scientists think that comets bombarding the young Earth 4 billion years ago brought the vast quantities of water that now fill Earth's oceans. Others have cast doubt on this idea. The detection of organic molecules in significant quantities in comets has led to speculation that comets or meteorites may have brought the precursors of life--or even life itself--to Earth. In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the amino acids that make up proteins through shock synthesis. The speed at which comets enter the atmosphere, combined with the magnitude of energy created after initial contact, may have allowed smaller molecules to condense into the larger macro-molecules that served as the foundation for life.

Most comets have two tails, which appear as the comet approaches the Sun. Solar wind and the sun's magnetic field sweep dust particles from the coma into a long tail that stretches behind the comet. A second tail is made of electrically charged molecules of gas (called ions). Very rarely a comet will have a third tail made of sodium, which we usually don't see with our unaided eyes.

The Ulysses spacecraft unexpectedly crossed the Hyakutake comet's tail at a distance of more than 500 million kilometers from the nucleus, making it the longest tail ever measured.