From Gretchen Siegchrist,
Your Guide to Desktop Video.
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Definition: :

A media container format is a format that can contain several types of data, such as audio and video. The video file formats we’re familiar with, such as Quicktime movies (.mov), are media container formats. They contain data that has been compressed using a video compression format or codec.

Some container formats just contain audio, like WAV files for Windows or AIFF files for Macs. Others contain audio and video, such as ASF files for Windows, which contain audio compressed with the WAV codec and video compressed with the WMV codec. Still others are exclusive to still images.

There are dozens of these container formats. If you’re uploading a video to an online sharing site, check to see what formats the site supports. Sometimes this can be confusing because the list of accepted formats may have both compression formats like MPEG-4 and container formats like .mov listed. All this means is the site accepts files compressed with those codecs in those final file formats.