A bong is a filtration device/apparatus generally used for smoking cannabis

Cannabis (drug)

Cannabis, also known as marijuana among many other names, refers to any number of preparations of the Cannabis plant intended for use as a psychoactive drug or for medicinal purposes. The English term marijuana comes from the Mexican Spanish word marihuana...

, tobacco

Tobacco

Tobacco is an agricultural product processed from the leaves of plants in the genus Nicotiana. It can be consumed, used as a pesticide and, in the form of nicotine tartrate, used in some medicines...

,or other herbal substances.

In construction and function a bong is similar to a hookah

Hookah

A hookah A hookah(Gujarati હૂકાહ) A hookah(Gujarati હૂકાહ) (Hindustani: हुक़्क़ा (Devanagari, (Nastaleeq) huqqah) also known as a waterpipe or narghile, is a single or multi-stemmed (often glass-based) instrument for smoking in which the smoke is cooled by water. The tobacco smoked is referred to...

, except smaller and more portable. A bong may be constructed from any air- and water-tight vessel by adding a bowl and stem apparatus (or slide) which guides air downward to below water level whence it bubbles upward ("bubbler") during use.