MKV (Matroska Video) is an open-source multimedia container format that can hold virtually any combination of video, audio tracks, subtitle tracks, and chapter markers. Named after the Russian Matryoshka nesting dolls, MKV is extremely flexible — it supports multiple audio tracks in different languages, multiple subtitle formats, chapter navigation, metadata, and thumbnails, all within a single file.
MKV is the preferred format in the media enthusiast community for storing high-quality video with multiple audio/subtitle options. Blu-ray rips typically use MKV for this reason. The format also supports lossless codecs (FLAC audio, lossless video) and very high resolutions without restrictions. MKV files can contain practically any video codec including H.264, H.265, VP9, AV1, and even DivX.
The limitation of MKV is compatibility: it doesn't play natively on iPhone, iPad, or many Smart TVs. VLC and Plex handle MKV well on most platforms. For sharing and streaming, convert MKV to MP4 (if the contained codec is compatible, this can be done quickly by remuxing). FileCurve can compress MKV video files.