Comets have very elliptical orbits. This means that they go around the Sun in a large, stretched out oval, rather than a circle. The furthest point from the Sun in an orbit is called aphelion. The aphelion of most comets is way beyond the planet Neptune. The closest point from the Sun in an orbit is called perihelion. The perihelion of some comets can be closer to the Sun than the planet Mercury!
Comets' orbits take them far away from the Sun and then back towards it. Every time a comet gets close to the Sun it gets smaller as some ice "boils" off. This means if a comet passes close to the Sun too many times it will eventually break up into pieces!
The main, central part of a comet is called its nucleus. During most of their orbit, comets are hard to see. This is because they are small and made of dusty ice so do not reflect much light from the Sun. As a comet get closer to the Sun, the ice in the comet heats up. Some of this ice turns into a gas. The gas (plus dust and some water) forms a cloud around the nucleus, called a coma. The coma gets lit up by the Sun's light making it easier to spot with a telescope.
The Sun's solar wind also pushes the coma into a large tail which stretches out from the comet. This is why this tail always points away from the Sun. The gas in the tail contains changed particles called ions. This is why this tail is sometimes called the 'ion tail'. Some comet's ion tails are over 1 million km long!