VDF files can often be seen when installing games distributed through Valve's Steam game client, such as Half-Life 2 and Counter-Strike. The file features Valve's KeyValues format, which is a plain text format that saves key-value parameters in a tree hierarchy. The format is similar to JSON and YAML. While VDF files are meant to be used by the Valve Source game engine, you can open the files with a text editor to view the contents