The Sumerians, like many other early civilizations, worshiped a variety of different gods. These gods changed over time and had many different jobs. Also the gods had certain cities they looked over. For instance the god Enlil watched over the city of Nippur. Gods also changed over the years. As the civilization aged and certain cities lost or gained importance so did the gods that were tied to them.
Other Sumerian Beliefs
The Sumerians believed that they lived in a massive dome. The dome contained the sky, moon, stars, and sun. They also believed that under the Earth was a dark neatherworld which contained demons and other creatures. Its not crazy to assume that this belief could have been the foundation and the beginning step in many religions conception of the underworld or Hell.