One way to think about a datum is as a set of spatial information that acts as a foundation for other data, just like concrete acts as a foundation for the structure of a building, including all the furnishings and decorations inside.Without a datum there would be nothing to securely support other spatial information, such as digital elevations, land use, or population. There are two types of datums, horizontal and vertical, that make up the National Spatial Reference System (NSRS). Scroll down this page
to see a representation of how geodesists help others construct spatial information into a map, like designing or decorating the interior of a building, by providing a solid foundation. Animated version.