A geodetic datum is a reference framework used to measure and represent the Earth's surface. It defines the size and shape of the Earth, allowing for accurate mapping and navigation. Different datums can vary based on the region and the specific measurements used, which is why it's important to choose the correct one for a given area. There are two main types of datums: horizontal datums, which provide a framework for latitude and longitude, and vertical datums, which measure elevation. Common examples include the WGS84 datum used by GPS and the NAD83 datum used in North America.