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 near Rome, between the 2nd and 3rd milestones of the Appian Way, where stone for buildings was quarried. The abandoned quarries were then used by the early Christians to bury their dead including the apostles Peter and Paul. They became the oldest known Roman catacomb and were much later named San Sebastiano. After the name catacombs was used for this place, it was soon used for all subterranean, mazelike, antique graves in Rome. Today the definition of catacombs is often simplified to underground tunnels with compartments dug into the sides to hold graves. Sometimes the word catacombs is used for cellars of unknown origin and usage, but it is always used for artificial caverns.
So catacombs are underground places for burials, but most of them were actually not dug for this purpose. The reason why catacombs were dug originally are numerous, cellars, storage rooms, cave houses, quarries 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 dead underground. This was the only possibility to bury them according to the Christian believes. The Christians used the caverns to hide and to live, and most important to worship god, but the word catacomb became a synonym for underground graves.
Today many catacombs in Europe are going back to the times of the plague, the black death pandemics, when so many people died, that they were buried in piles everywhere possible. Pandemics also caused the need to bury people before the bodies of the former owners of the graves were completely gone. As a result the graves were opened and the bones removed. They were stored in bone houses, which were often a basement of a chapel or church. Those bone houses were not used for burials, they were only used to store the bones after the exhumation or disinternment. Such bone houses are called ossuary, but again the term intermingles with the term catacomb. For example the most famous catacombs of the world, the catacombs of Paris, are actually an ossuary. That is why we have not separated the two terms.
Bone houses were especially popular during the Baroque. The MEMENTO MORI (Remember the death) was quite popular at this time. People were told to always remember their death, to make them live responsible. However, this also led to human bones being painted or otherwise decorated, or to sculptures, symbols and even candelabras being formed from the bones. Some artists have even signed their work with human bones.