Container format

The container decides how the streams are packaged and labelled inside one file, and the codec decides how the picture and sound were compressed. An MP4 can hold H.264 or H.265, and a MOV can hold ProRes or that same H.264, which is why two files with the same extension can behave completely differently. This trips people up on delivery. A client asking for MP4 usually means H.264 inside an MP4, and an editor handed a MOV full of ProRes needs the disk space and the machine to match. When something will not play, the container is rarely the problem. The codec inside it usually is.