Kate is a codec for karaoke and text encapsulation for Ogg. Most of the
time, this would be multiplexed with audio/video to carry subtitles,
song lyrics (with or without karaoke data), etc, but doesn't have to be.
A possible use of a lone Kate stream would be an e-book. Moreover, the
motion feature gives Kate a powerful means to describe arbitrary curves,
so hand drawing of shapes can be achieved. This was originally meant for
karaoke use, but can be used for any purpose. Motions can be attached to
various semantics, like position, color, etc, so scrolling or fading
text can be defined.
More information can be found at http://wiki.xiph.org/index.php/OggKate