No matter what their size and shape, all rivers are moving water from one place to another!
There are many different types of rivers. Some snake back and forth. Others are straight. Some flow fast. Others flow slowly. The longest river in the world is the Nile in Africa, which flows north into the Mediterranean Sea. Many rivers are very small. They are called streams.
Rivers and streams erode the land, making valleys grow deeper over many years by carrying sand and silt downstream.