What city is like unto this great city...that great
city that was clothed in fine linen and purple and
scarlet and decked with gold and precious stones and
pearls!...Babylon the Great, the Mother of Harlots
and of the Abominations of the Earth.
-Book of Revelation

Babylon was a city located on a branch of the river Euphrates. The language was a form of Akkadian, and written in the cuneiform script developed by the Sumerians.

The oldest settlement date is uncertain; in the 3rd millennium, while Sumer was at its peak, and while Nineveh and Ur were active on the international scene, it was nothing more than a farming community. It was at the end of that millennium that Babylon overtook a major role, due to a happy coincidence of a power vacuum and trade, taking advantage of its location at the spot where the Tigris and Euphrates came closest together.

Around the time of the king Sargon, shortly before Hammurabi, it codified its laws and began to organize the settlements around it and the labour necessary to irrigate the dry but fertile soil. Soon after Hammurabi, the southern states revolted, and after several years of turmoil was given the final blow by Hittite raiders, around 1500 B.C. A Dark Age followed, where little is written and thus even less known.

Babylon never again gained political importance, which was assumed in turn by the Kassites, the Assyrians, the Persians, and the Greeks. During that whole time, though, Babylon remained a cultural center. Its gods, most notably Marduk, were held in high esteem, and Assyrian kings tried to portray themselves as friendly to the Babylonian temples. In addition, it kept its language and writing, and was viewed as a source of learning. It was this legacy, of systematized education, literature, trade, and religion, that preserved the city until the end of the first millennium.

A few references, in case anyone wants to know more or prove my overly generalized writeup wrong:

Oates, J. Babylon. (London 1986).
Oppenheim, A.L. Mesopotamia: Portrait of a Dead Civilisation. (Chicago 1964).
Brinkman, J.A. A Political History of Post-Kassite Babylonia. (Rome 1968)