A short primer on video-files, formats and codecs – because it is often cause for confusion:
A video file is a container. It usually contains one video-track and one or more audio-tracks.
How these tracks are stored in the file is defined by the file-format. Common formats are avi, mov, ogg, mkv, mpeg, mpeg-ts, mp4, flv, vob
Each of the tracks by itself in encoded - using a Codec. Common Video-Codecs are h264, mpeg2, mpeg4, theora, mjpeg, wmv3. Audio-Codecs: mp2, mp3, dts, aac, wav/pcm.
Not all codecs can be packed into a given format. For example the 'mpeg' format is limited to mpeg2, mpeg4 and mp3 codecs (not entirely true). DVDs do have stringent limitations as well. The opposite would be .avi; pretty much every audio/video codec combination can be contained in an avi file-format.
To make things worse, naming conventions for video codecs and formats are often identical (esp mpeg ones) which leads to confusion. All in all it is a very wide and deep field. Suffice there are different uses for different codecs and formats.
Ardour supports a wide variety of video file-formats and video-codecs. More specifically, Ardour itself actually does not support any video at all but delegates handling of video files to ffmpeg which supports over 350 different video-codecs and more than 250 file-formats.
When importing a video into Ardour, it will be transcoded (transcoding: change from one format and codec to another) to avi/mjpeg for internal use (this allows reliable seeking to frames at low CPU cost - the file-size will increase, but hard-disks are large and fast).
The export dialog includes presets for common format & codec combinations (such as DVD, web-video,..). If in doubt use one of the presets.
As last note: Every time a video is transcoded the quality can only get worse. Hence for the final mastering/muxing process, one should always to back and use the original source of the video.