An m3u8 file is a UTF-8 text manifest listing media segments and metadata for an HLS player to consume. Each line is either a comment, a tag starting with #EXT-X-, or a URL pointing to a media segment. Tags carry duration (#EXTINF), discontinuity markers (#EXT-X-DISCONTINUITY), encryption keys (#EXT-X-KEY), partial-segment definitions (#EXT-X-PART), and dozens of other annotations.
There are two kinds of m3u8 in HLS. The multi-variant playlist (sometimes called master) lists available renditions — different bitrates, resolutions, codecs and languages — each referenced by its own URL. The media playlist (one per rendition) lists the actual segments. The player fetches the multi-variant playlist first, picks a starting rendition, and then keeps re-fetching the media playlist (for live) or reads it once (for VOD).
m3u8 evolved from the older M3U format used by audio players (Winamp, iTunes), which is why the file extension and the basic syntax look pedestrian compared to DASH's XML MPD. The simplicity is a feature: an m3u8 can be inspected with cat, generated with printf, and concatenated trivially. Almost every text editor highlights it, every CDN serves it as application/vnd.apple.mpegurl, and every HLS-capable player parses it.
