EPSG stands for European Petroleum Survey Group and is an organization that maintains a geodetic parameter database with standard codes, the EPSG codes, for coordinate systems, datums, spheroids, units and such alike. Additionally, this database contains the parameters for these objects or - if they cannot easily be expressed as values - at least references to where such parameters can be found.
Every geographic object (coordinate system, spheroid, unit etc.) gets assigned a unique number. The database is under active maintenance.
The coordinate system with its EPSG code is usually stored in the source file (GeoTIFF, Shapefile, LandXML, ...) and is read automatically by Virtual Surveyor. Virtual Surveyor allows to set a Project Coordinate System through the EPSG code. In the example below it is 31370. The terrain data will be reprojected from their source Coordinate System to the Project Coordinate System.
An online database with all cooordinate system and its EPSG code can be found online on http://spatialreference.org.