Code Geass

[Yukimura]

In case you're wondering CCCP = Combined Community Codec Pack . Your demonstrated lack of familiarity with the majority of the fansub scenes inner workings leads me to believe you would benefit from having this explained in more detail.

The mkv container is a superior video container format to the avi because it supports significantly more features, arguably the most appealing being the H264 codec.

A codec is what is used to compress/decompress video and the H264 is better than XviD (what most avi's use) at packing more data (aka more quality) into a similar size. Thus more and more scan groups are releasing in mkv format because they can make a better product than if they used avi. The combined community codec pack is meant to make it easier for people with 0 knowledge of codecs or the like to enjoy the highest quality video possible, once you install it it will take care of everything for you and allow even WMP11 (a somewhat poor and very much behind the times media player) to play mkvs, and thus H264 encoded video. But, since you mentioned flv I get the feeling you are used to watching youtube videos, which have horrible quality. If that never bothered you then you might just not care about video quality at all, still you should get the CCCP because the groups that release in mkv also tend to be the best groups in terms of overall fansub quality.