AVI, MP4, MOV and MKV are all video containers — wrappers around the actual encoded video and audio. MP4 is the universal safe choice for sharing and playback; MOV is Apple's format; MKV is flexible but poorly supported on phones and TVs; AVI is old and inefficient. When in doubt, convert to MP4.
Container vs codec — the key distinction
These extensions (.avi, .mp4, .mov, .mkv) are containers; the video inside is encoded with a codec such as H.264, HEVC, or AV1. Two files with the same extension can behave differently depending on the codec inside — which is why "will it play?" is really about the codec and the container together.
MP4 — the universal default
H.264 video in an MP4 container plays on essentially every device, browser, and app, at good compression. It's the format to share in and the safest target when something won't play. Convert to MP4 if you're unsure.
MOV — Apple's format
QuickTime's container, and what iPhones and Macs record to. Great inside the Apple world, but it often won't play on Windows without the right codec. Convert MOV to MP4 for cross-platform sharing.
MKV — flexible but poorly supported
Matroska can hold multiple audio tracks, subtitles, and almost any codec, which makes it popular for downloaded and archived video. The downside is that iPhones and many smart TVs won't play it. Convert MKV to MP4 to play it on those devices.
AVI — old and inefficient
One of the oldest containers. It still works, but it compresses poorly (large files) and lacks modern features like proper streaming and subtitle support. Convert AVI to MP4 for smaller files and far better compatibility.
Which should you use?
For sharing and everyday playback, MP4. Keep MOV or MKV only if you're staying in an ecosystem that supports them or you need their specific features (Apple editing, MKV subtitles). When a file won't play, converting to MP4 is almost always the fix — see why won't my video or audio file play.