There are several things that define what a civilization is. The most important are language, culture, an agricultural surplus, and art that a civilization creates. Why are these things important to defining a civilization? No group of people can get along without food, culture, the ability to communicate, and some sort of artistic relics that future people can find. These aspects define what a civilization is, and what it contributed to the history of humanity.
First off, the agricultural surplus is the most important thing that a civilization can have. Why is this? All people need food, and if they don't make enough of it, then they have to go find more of it. If they spend all of their time looking for food, what else do they have time to do? The surplus allows some people in the group to do other things besides look for food, and they can become specialized.
These people that go off and do other things besides look for food influence the culture of the people. Priests can help guide the spiritual beliefs of a people, and those beliefs can only come if people are allowed to be curious about the world around them. Artisans can make crafts, and other goods that people can use to decorate, or to marvel at. Governments can be established, and a rule of law can be established, which is also very important. People can do all the other things that make a society run, and creates a civilization.
If these people don't have language though, you can forget about having a civilization. If you can't communicate, how can you write down your laws? How do you write down any copies of your scientific work? While very few manuscripts are available from that time, it would seem to be beneficial to have some sort of record keeping system so that the people can keep track of what they learn. It also comes in handy when collecting taxes, or passing down religious texts from generation to generation.
Art defines a people. When you see Michaelangelo, and you see Da Vinci, you see the manifestation of the time period they lived in. When you see a Middle Age tapestry, you see the time period that they lived in. You see the depictions of what people believed, what they liked to do, and the culture around them. You see what was taboo, and what was considered just a part of the culture. The Greeks would always carve perfect bodies on their sculptures, regardless of how the person actually looked. The Romans didn't do this, and you can see the culture through art.
These attributes, and more define what a civilization are. These seem to be so basic, yet they are so vital to the sustaining of any civilization, and allowing any peoples to be called such. Any group of people can be farmers, or herders, but it takes a lot to become a distinct civilization.