The default reference system is WGS-84 with coordinates in degrees of longitude and latitude on Earth. The SRID number for this is 4326 and is assigned by default to all new geometries. ST_Transform can be used to re-calculate coordinates of a shape from one spatial reference system to another. ST_SetSRID can be used for altering SRID without altering coordinates.

Distances and precisions for "latitude-longitude" spatial reference systems are in kilometers, as approximately calculated with the haversine formula. For other systems they are in the same unit as the coordinates.