EPSG codes are numeric codes associated with coordinate system definitions. For instance, EPSG:4326 is geographic WGS84, and EPSG:32611 is "UTM zone 11 North, WGS84". The WMS protocol uses EPSG codes to describe coordinate systems. EPSG codes are published by the OGP Surveying and Positioning Committee (http://www.epsg.org/). A list of PROJ.4 definitions corresponding to the EPSG codes can be found in the file /usr/local/share/proj/epsg.