A projection (.prj) file is a text file containing information about the coordinate system and map projection. A projection file can be used to extend the properties of a CAD drawing to include a spatial reference. The information is stored in the well-known text (WKT) format.
A geographic coordinate system (GCS) includes information about the datum, spheroid, prime meridian, and angular unit of measure, usually in degrees. In addition to the GCS information, a projected coordinate system (PCS) defines additional parameters such as false easting, false northing, central meridian, scale factor, latitude of origin, and linear units, usually in meters or feet.
The contents of a projection file look similar to this:
PROJCS["NAD_1983_NSRS2007_StatePlane_Kansas_South_FIPS_1502_Ft_US",GEOGCS["GCS_NAD_1983_NSRS2007",DATUM["D_NAD_1983_NSRS2007",SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Lambert_Conformal_Conic"],PARAMETER["False_Easting",1312333.333333333],PARAMETER["False_Northing",1312333.333333333],PARAMETER["Central_Meridian",-98.5],PARAMETER["Standard_Parallel_1",37.26666666666667],PARAMETER["Standard_Parallel_2",38.56666666666667],PARAMETER["Latitude_Of_Origin",36.66666666666666],UNIT["Foot_US",0.3048006096012192],AUTHORITY["EPSG",3543]]