M2TS (MPEG-2 Transport Stream) is a Blu-ray BDAV container format that uses MPEG-2 transport stream and comes with a .m2ts filename extension. It can manipulate audio, video, and other metadata such as disc menu and subtitles. At a bare minimum, any Blu-ray player should play M2TS files easily.
FLAC (Free Lossless Audio Codec) is a free lossless digital audio format developed by Xiph.Org. A compressed FLAC file can be reduced by between 50-70% of the original size and decompress back to the original sound data. This file format supports metadata like ID3 tags, cover art, and album title.