Certain conglomerations of humans are seen as civilizations, whereas, say, nomadic cultures generally aren't, unless you are, say it with me, the Mongols. By calling some groups civilizations, you imply that all other social orders are uncivilized, which is basically just another way of saying that they're savages or barbarians. Side note, originally Greek, the word barbarian denoted anyone who did not speak ancient Greek, because to the Greeks, all other languages sounded like bar-bar-bar-bar-bar-bar-bar. So that is to say that we are all essentially barbarians, except for the classics majors, which is worth remembering when we're discussing civilizations. Civilizations are like most of the things we like to study. They're intellectual constructs. No one woke up in the city of Thebes in Egypt one morning and said, what a beautiful morning, I sure am living at the height of Egyptian civilization.