Understanding GeoTIFF tags?

Here's an extremely dense but useful explanation of the GeoTIFF specification: http://duff.ess.washington.edu/data/raster/drg/docs/geotiff.txt

The PCS tag is the EPSG number of the projection coordinate system. GCS is the EPSG number of the geographic coordinate system. So your first file is in UTM 59S, with linear unit of meters. The second file is World Mercator, also with linear unit of meters. The cell size of both is ~0.597m. GeogAngularUnitsGeoKey is the unit of the GCS, and ProjLinearUnitsGeoKey is the unit of the PCS.

Hopefully that will get you started.