The ancient Sumerians developed a written language called cuneiform.
It began as pictographs, pictures of things that acted as words. Pictographs worked, but hey were rather cumbersome. Soon, the clever ancient Sumerians started to use wedge-shaped symbols for objects and ideas instead of pictures.
We know a great deal about the ancient Sumerian civilization from the written records they left behind. They kept excellent records and lists of things. They listed their household goods. They listed their court activity. They listed their sales and purchases. They even kept a list of their kings that was updated from time to time, as new kings came to power. The Sumerians never invented paper or ink, so they used tools made of wood or stiff reeds to press the symbols into clay tablets. Many thousands of these tablets have survived the ages and we can see them today. Some are worn, some are broken, but many thousands have been found complete. The largest collection of these was found at the Library of Nineveh.
Cuneiform became the written language from as early as 5000 BC. As the civilization of Sumer started to decline, other civilizations continued to use the Sumerian written symbols. Thanks to the Sumerians, we know a great deal about the ancient Mesopotamian civilizations of Babylon and Assyria as well.
Ultimately, the use of cuneiform died out and was replaced with other written languages. When modern language experts tried to read cuneiform, they found cuneiform very puzzling. It had been thousands of years since anyone had written or read cuneiform. No one in modern times remembered what the symbols meant. But things changed when the Language Detective, Henry Rawlinson, a British officer in the 1800s, went hiking in the countryside in Persia (now called the country of Iran.) He spotted a picture, carved into the rock of a high cliff. This picture told the story of an ancient king, his servants, and some prisoners. The story was told in pictures, in cuneiform, and in another writing similar to an East Indian language that Henry Rawlinson already could read and write. The same story was told in three different ways so that most probably, in ancient times, everyone could understand it. That was the break scientists needed. Once they understood a few words in cuneiform, they could build on that and start recognizing the same symbol on tablets and documents. They could guess at the meaning of other symbols around the symbols they understood. They worked very hard at it until they could read a great deal of cuneiform. (The same sort of thing happened in Egypt, when they found the Rosetta Stone, which helped scientists understand ancient Egyptian hieroglyphics.)
Cuneiform is still not easy to read since the meaning of some of the symbols changed over time, as language changed. But thanks to the Language Detective, and a carving on a cliff, with patience and effort, it can be deciphered. There are many cuneiform tablets that are still waiting to be translated, and more ancient tablets still being found today. The more scientists translate, the more we learn about the ancient Sumerians. They were such a clever people. They not only invented writing, they also invented the first sailboat and the wheel - three very important inventions in ancient times, each having a huge effect on the culture of these ancient people.