Catacombs are ancient underground cemeteries; the catacombs here in Rome were mainly used by Christians (and occasionally Jews) from the 2nd to the 5th centuries AD. At the time underground burial was popular due to the high price of land, and for the sense of community it gave. For persecuted Christians in pre-Christian Rome, underground burial meant they were free to worship their Biblical martyrs and prophets, and to display Biblical iconography (a big no-no in the Roman era).
Because burial within Rome's walls was forbidden by law, all of the city's catacombs are located on the outskirts of modern Rome, typically along major roadways (two ancient catacombs, for example, are located along the Appian Way).
There are five major catacombs in Rome that are open to visitors. St Sebastian and St Callixtus catacombs are both on the Appian Way (via Appia Antica). The catacombs of St Agnes are on Via Nomentana, those of Priscilla are on via Salaria, and the catacombs of Domitilla are on via delle Sette Chiese.