The pomerium was the sacred boundary of the city of Rome. Legally speaking, Rome only existed within the pomerium; everything beyond it was simply land belonging to Rome. Tradition maintained that it was inaugurated by Servius Tullus, but it did not follow the line of the Servian walls, and it is unlikely that he actually did establish the sacred boundary, which remained unchanged until the dictatorate of Lucius Cornelius Sulla.
It was not a wall but rather a legally and religiously defined one marked by white stones called cippi, and it did not encompass the entire metropolitan area (the Palatine was within the pomerium, but the Capitoline and Aventine were not). The Curia Hostilia and the well of the Comitia in the Forum Romanum, two extremely important locations in the government of the city-state and its empire, were located within the pomerium. The temple of Bellona was beyond the pomerium.
Religious and political constraints forbade any anointed sovereign from entering the pomerium. As a result, visits of state were somewhat awkward; Cleopatra, for example, never actually entered the city of Rome when she came to visit Julius Caesar.
Furthermore, promagistrates and generals were forbidden from passing beyond it, and resigned their imperium immediately upon crossing it. As a result, a general waiting to celebrate a triumph was obligated to wait outside the pomerium until his triumph. The Comitia Centuriata, one of the Roman assemblies, was obligated to meet on the Campus Martius outside the pomerium. Pompey's Theater, where Julius Caesar was murdered, was also outside the pomerium and included a Senate chamber where the Senate could meet with the attendance individual senators who were forbidden to cross the pomerium and thus would not have been able to meet in the Curia Hostilia.