BIK
BIK files are the video files created using Bink, as part of Epic 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 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 Source 2004, was the first Source game to utilize Bink video for it's startup video.
By today's standards, Bink (retroactively Bink version 1.0), suffers from 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 
Mac ports of Source engine games may instead use QuickTime's MOV video file.
The 
Titanfall engine branch has been upgraded to use Bink 2, which uses the .bk2 file format instead, and has more features, such as supporting multi-core rendering, HDR, a full color range, and better video quality than Bink 1.
In post-panorama versions of the 
CS:GO engine branch, newer versions of 
Hunt Down the Freeman and 
Team Fortress 2 (x64_test beta branch), all 
Source 2 games, as well as 
Strata Source, it is replaced by WebM.
Todo: Expand on this more
