BIK
BIK files are the video files created using Bink, as part of RAD Video Tools. The startup video that plays on all Valve games (since the 
Source 2007 update) is a .bik file. They are also used in the menu backgrounds and the elevator screens in 
Portal 2. Prior to Bink being added to Source, previous games or older version of those games uses AVI file instead.
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.
In some 
Mac port of Source engine games, it may instead use QuickTime's MOV video file.
In all games using the 
Titanfall branch, 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 
CS:GO engine branch (which uses Panorama) and newer version of 
Hunt Down the Freeman, all 
Source 2 games, as well as 
Strata Source, it is replaced by WebM.