NAD83 (North American Datum of 1983) is the official horizontal geodetic datum for North America, defining the reference frame used to assign latitude and longitude (and projected coordinates) across the United States, Canada, and Mexico. It is realized through the GRS80 ellipsoid and is the basis for most authoritative spatial data on the continent.
Why it matters
Almost every government and survey dataset in North America — USGS topographic data, state plane coordinate systems, county parcel layers, NRCAN products — is referenced to NAD83 or one of its realizations. If your GIS layers use NAD83 and you overlay them with WGS84 data without accounting for the difference, features will be misaligned. In the conterminous US the offset between NAD83 and WGS84 is typically on the order of 1–2 meters, which is irrelevant for small-scale mapping but significant for surveying, engineering, and precise asset location.
A concrete example
Common EPSG codes you will encounter include EPSG:4269 (geographic NAD83, latitude/longitude) and the NAD83-based UTM and State Plane zones, such as EPSG:26910 (NAD83 / UTM zone 10N) or EPSG:2229 (NAD83 / California zone 5, US feet). NAD83 has multiple realizations (e.g., NAD83(2011), NAD83(CSRS) in Canada), and these can differ from one another by centimeters to a decimeter — important for high-accuracy work.
Common pitfall
People often treat NAD83 and WGS84 as interchangeable because they were nearly identical at definition in the 1980s. They have since diverged because NAD83 is fixed to the stable North American tectonic plate while WGS84 tracks the global ITRF. For sub-meter work, always apply a proper datum transformation rather than assuming the two are equal.