The name catacomb is derived from the Greek kata kyumbas (at the gorge), which was romanized to ad catacumbas. What the Romans meant was a valley, with the oldest known Roman catacomb S. Sebastiano. The subterranean, mazelike, antique graves at Rome were called catacombs. Today the definition of catacombs is often simplyfied to underground tunnels with compartments dug into the sides to hold graves.
The reason why catacombs were dug originally are numerous, cellars, storage rooms, cave housings and much more. During the Roman Empire, at the time when the Christians were pursued, they used the catacombs as an hideout. They built their churches underground and buried their deads underground. This was the only possibility to bury them according to the Christian believs.
The Christians used the caverns to hide and to live, and most important to worship god, but the word catacomb became a synonym for undergound graves. Today many catacombs in Europe are going back to the times of the pest, when so many people died, that they were buried in piles everywhere possible.
Sometimes the word catacombs is used for cellars of unknown origin and usage, but it is always used for artificial caverns.