VPK - Valve Pak is a simple game file that is mainly used for the collection of data files. It can also be used for map storing and other game content. There is a directory that is also contained in many of the VPK files that you can take advantage of also. If you want to make game mods, this is the extension you should install on your computer. Before you install it, you will be aware of the different file names that you can access so you know what to look for prior. Again, this is a master directory file and should be used only if you are knowledgeable of these other programs that are supported through the extension. It may take a while to get used to, but you will get the hang of it eventually.