PAK

PAK file formats are package formats previously used by GoldSrc engine games to store content, which was first originated in Quake, this was later changed during the WON/Retail to Steam transition throughout 2003-2004, which the game file is now stored inside .gcf file, then later in 2013, with SteamPipe, the game file is now stored directly to the filesystem (example: SteamApps\common\Half-Life\valve), unlike Source and Source 2 engines (which continued to packing game files inside .vpk file) as most modern systems are fast enough to load GoldSrc game files quickly.