Italy
Naples and Sicily
Earlier in the Middle Ages, the south end of Italy had been ruled by the Byzantine Empire, from the sixth to the eighth century. Sicily was conquered by the Muslims in the eighth century, and they managed to gain some outposts on the mainland as well. In the eleventh century, though, the Normans entered the scene and by 1100 they had driven the Muslims out of Sicily and the Byzantines out of the southern mainland. To legitimize their rule they turned to the papacy: one pope made the south a duchy, and another made it a kingdom (1130). The complex relationship between the Kingdom of Naples and the papacy existed from the start: there was a split in the papacy around 1130 and Roger supported Anacletus, who raised the duchy to a kingdom in return for Roger's support. The newly-minted king then had to switch sides and support Pope Innocent II (who at length triumphed over Anacletus) in order to retain his title (1139).
Il Regno
Over the centuries "The Kingdom" (Il Regno) had several different titles, including the kingdom of Naples, of Sicily, of Naples and Sicily, and even "Trinacria" (a Renaissance invention). The shifts in title reflect shifts in real power, for at times Sicily and Naples were united under a single ruler but at times were separate. The two when not united were usually ruled by different branches of the same family, though, so there is a sort of continuity. The fundamental difference between the two areas was that Sicily had a strong Muslim influence whereas the mainland had a strong Greek influence. Also, speaking very broadly, the great barons of Naples (this word usually will mean the kingdom rather than just the city) were more independent and more powerful than those of Sicily; and Naples tended to get more tangled up in the affairs of central and northern Italy than did Sicily.
The city of Naples was the most important urban center in the Kingdom, with Palermo on the island of Sicily being the second. Other centers included Taranto and Messina. Most of the south, though, was extremely rural, even wild. Sicily, in particular, had long been a center of grain production, while the mainland was mountainous and produced little for export.