Each fan-sub releases their anime in 2 types of files, .mkv and .avi Each having their own pros and cons. Here is generally what they are:
.mkv eplanation - This video format renders the audio, video and subtitles in 3 different parts. Thus explaining the lag people might get from watching vids with this format. Fan-subs like releasing this format first cuz they can easily edit the subtitles if there is a problem.
.avi explanation - All of the video's contents will be compressed into 1. Basically audio, video and subtitles are merged and less memory will be used by the com.
.mkv Pros - Generally higher resolution. - Somewhat better sound quality. - Usually released earlier.
.mkv Cons - Might lag noob coms. Cuz the audio and video are 2 diff programs. - The file size might be larger than .avi