What is a Video Container Format?
TL;DR: A video container (MP4, MOV, MKV, WebM, AVI) is a file format that wraps together video, audio, subtitles, and metadata into a single file. The container is separate from the codec — the same H.264 video can be in an MP4, MOV, or MKV container.
MP4 is the most universal container — supported by virtually every device and platform. MOV is Apple's container with similar capabilities. MKV is open-source and supports unlimited tracks. WebM is Google's web-optimized container.