What is codec?
The algorithm that compresses video or audio data - H.264 and AAC are the two you'll run into most, packed inside a container like MP4.
A codec is not the same thing as a file format: MP4 and MOV are containers, while H.264, HEVC, VP9 and AV1 are the codecs that actually compress the picture inside them. The same container can hold different codecs, which is part of why two .mp4 files can behave completely differently on upload.
The codec is what determines hardware decoding support (does this phone's chip know how to play it instantly), compression efficiency (how small a file gets at a given quality) and editing compatibility - H.264 is decoded natively on essentially everything; newer codecs trade some of that universality for smaller files.
Koldflux picks the right codec, container and bitrate for every platform automatically - this glossary exists because our own team had to get it right first.
Automate this in Koldflux