A Video plays a specific AssetVideo. The video can be controlled by left clicking on it (stop and play). You can specify whether the video should loop once playing, and whether the video should start playing automatically when the room is entered. Multiple Video's can be associated with one AssetVideo without any extra performance penalty (useful if you want the same video to appear at multiple locations in the room). The video will appear in the room as a rectangle, and the ratio of the height and width dimensions will match that of the video itself, preserving aspect ratio. All videos in a room are stopped automatically when the user leaves the room.
http://janusvr.com/docs/build/roomtag/index.html#Video