BIK files are the video files created using Bink, as part of Epic Video Tools. The startup video that plays on all Valve games, and third-party games (since the update) is a .bik file. They are also used in the menu backgrounds and the elevator screens in . Prior to Bink being added to Source, previous games or older version of those games uses AVI file instead. One game, , which runs on Source 2004, was the first Source game to utilize Bink video for it's startup video.
By today standards, Bink (retroactively Bink version 1.0), suffers from large file sizes while having slighting lower quality, and it's was much more complicated when it's come to converting files, such as AVI or MP4 (with deprecated QuickTime application) to BIK that may ended up causing audio/video desync or other issues. Due to these issues along with many others, Bink 2 was created, which have massive improvement over Bink 1. Furthermore, there are alternatives to Bink video, such as Google WebM, which is open-source and completely free.
In some MOV video file.
In all games using the , the engine has been upgraded to use Bink 2, which uses .bk2 file format instead and have more features, support multi-core rendering and HDR, full color range, and better video quality than Bink 1 had.
In the newer version of (which uses Panorama) and newer version of , all games, as well as , it is replaced by WebM.