.PRJ projection definition files

.prj files define the coordinate reference system for a set of geodata (e.g. a GeoTIFF or ESRI Shapefile). The files contain plain ASCII text and can be edited with any text-editor (e.g. notepad++).

Inside the .prj file, a textual definition of the projection in the OGC WKT format is expected.

If you know the EPSG-Code for a geodataset - but not the WKT definition - you can look it up at spatialreference.org and download the WKT definition there as a .prj file. This for example, is the .prj file for EPSG:4326.