In Detail:
MythVideo
A place to store, organise and play all those random video clips. Supports the MythTV internal player as well as several different external player utilities, so YouTube, Flash, archived DVDs, recorded TV or video camera clips should all be no problem.
Also used to play DVD-Videos and VCDs from your frontend's CD/DVD drives, and when used with the MythTV internal player, allows bookmarks to be saved so you can continue watching from where you went to dinner, bed, the pub, etc.
Read more in the MythTV Wiki entry for MythVideo