BIK
BIK files are the video files created using Bink Video, a part of Epic Video Tools, originally RAD Video Tools.
The startup videos that play on all Valve and third-party games (since the 
Source 2007 update) are .bik videos. They are also used in the menu backgrounds in Left 4 Dead, Left 4 Dead 2, Alien Swarm, Portal 2 and the elevator screens in Portal 2. Prior to Bink being added to Source, previous games or older versions of those games used AVI files instead. Vampire: The Masquerade - Bloodlines, which runs on beta version of Source 2004, was the first Source game to utilize Bink video for its startup video.
By today's standards, Bink (retroactively Bink version 1.0), are known for having large file sizes while having lower quality, lower framerate, and is much more complicated when it comes to converting files, such as AVI or MP4 (with deprecated the QuickTime application prior to RAD Video Tools update in late 2010s) to BIK, which ends up causing audio/video de-sync or other issues. Due to these and many other issues, Bink 2 was created, which has massive improvements over Bink 1. Furthermore, there are alternatives to Bink video, such as WebM, which is open-source.
Some Source games or ports may uses other file format instead:
Mac ports of Source engine games use QuickTime's MOV video file.- On
Xbox 360 prior to Left 4 Dead 2, it uses Windows Media Video (WMV) format, and on Nintendo Switch (Portal Companion Collection), it uses MKV. - The Titanfall branch has been upgraded to use Bink 2 (using BIK or BK2 format), which has more features, such as supporting multi-core rendering, HDR output, a full-color range, and better video quality & compression than Bink 1.
- In post-panorama versions of the CS:GO engine branch, newer versions of Hunt Down the Freeman and Team Fortress 2, all
Source 2 games, as well as Strata Source, it is replaced by WebM. - In The Stanley Parable it is replaced by IVF to avoid having to license Bink.