In the early part of 3rd millennium, three great civilizations developed nearly simultaneously on Nile, Euphrates & Indus rivers. We know a great deal about the first two because they have left us written records in the form of papyrus scrolls or long engravings on stones.
People of Indus valley, on the other hand, hardly left any written records except few inscriptions on the seals. So knowledge about Indus valley civilization is incomplete and subject to continuous updation.
Archaeologists call this civilization Harappa culture after the modern name of the place in Punjab located on the left bank of river Ravi. Meohenjo Daro, the second city, is located in Sind on the right bank of Indus river.
The culture was spread over 950 miles from North to South and includes large and small cities like Kalibanga in the valley of old Sarasvati river and many villages near Ropar on upper Sutlej up to Lothal in Gujarat. That this culture was same is proved from the use of bricks of same shape and size.
This was an truly Indian people civilization with no influence or migration from the Middle East. It was the continuation of early village culture. Each city had a well-fortified citadel. The uniformity in planning of streets, bricks and layout of the cities indicate a single centralized state rather than a number of free communities.