Video file formats
21 formats in this category.
Video formats are containers that wrap a video stream, one or more audio streams and metadata. The container (the extension) and the codec inside it are separate things, which is why two files with the same extension can behave differently.
Container versus codec
A video file has two separate parts: the container (the extension, such as MP4, MKV or MOV) and the codec inside it (such as H.264, H.265/HEVC or AV1). The container bundles the video stream, one or more audio streams, subtitles and metadata. The codec is the maths that actually compresses the pictures. This is why two files with the same .mp4 extension can behave differently: one may use a codec your device supports and another may not.
MP4 is the safe, universal container that plays almost everywhere. MKV (Matroska) is more flexible and popular for high-quality rips because it can hold many audio and subtitle tracks. MOV is Apple's QuickTime container, common out of macOS and iPhones. WebM is the open container built for the web.
Why a video won't play
When a video refuses to open, the container is rarely the problem; the codec usually is. Installing a capable player such as VLC, which bundles the codecs itself, fixes most cases without changing the file. If you need a file to play on a specific device, re-encoding it to H.264 inside an MP4 is the most compatible target.