Historically, two processing systems generated the USGS Landsat standard data products:
Level-1 Product Generation System (LPGS) - Currently, all Landsat data is processed through LPGS. LPGS metadata is contained in a MTL.txt file, which accompanies all data files.
National Land Archive Production System (NLAPS) - Some available Landsat 4 and Landsat 5 data were processed through NLAPS. NLAPS metadata is contained in a .WO file, which accompanies all data files. A listing of NLAPS-processed scenes can be found at L4-5TM_NLAPS.xlsx
While the products generated by the LPGS and NLAPS systems are similar, there are geometric, radiometric, and data format differences. The Comparison of LPGS vs. NLAPS Web page describes these differences.