A geodetic datum is an abstract coordinate system with a reference surface (such as sea level) that serves to provide known locations to begin surveys and create maps. In this way, datums act similar to starting points when you give someone directions. For instance, when you want to tell someone how to get to your house, you give them a starting point that they know, like a crossroads or a building address.
Geodesists and surveyors use datums to create starting or reference points for floodplain maps, property boundaries, construction surveys, levee design, or other work requiring accurate coordinates that are consistent with one another.
There are two main datums in the United States. Horizontal datums measure positions (latitude and longitude) on the surface of the Earth, while vertical datums are used to measure land elevations and water depths.
The horizontal datum can be accessed and used through a collection of specific points on the Earth whose latitude and longitude have been accurately determined by NOAA's National Geodetic Survey. One application of the horizontal datum is monitoring the movement of the Earth's crust. This type of monitoring is often used in places like the San Andreas Fault in California where many earthquakes occur.
The vertical datum is similarly "realized" through a collection of specific points on the Earth with known heights either above or below a nationally defined reference surface (e.g., mean sea level). Geodetic vertical datums are generally used to express land elevations. However, water level datums are a slightly different vertical datum, and are used as a reference level to which bathymetric soundings are referenced for nautical charts. Conversion between these two can be done through geodetic surveys at tide gauges.