One of the things that make comets interesting is the variation in appearance from comet to comet and even changes in the same comet over time. In a typical comet, the brightest part is the "head" which consists of a nucleus surrounded by a coma. The nucleus is the actual body of the comet and the coma is a ball of gas and dust surrounding it.

Both a gas tail and a dust tail are frequently visible. The greatest variation in the appearance of a comet is with the tail(s). The rate of release of dust and gas as well as the position of the Earth relative to the comet and the Sun determines the length and visibility of these features. The brightness of some comets, like Hale-Bopp, can rival  the brightest stars in the sky and have more than one bright tail. The majority of the dozen or so comets that enter the solar system each year are not visible with the unaided eye.