currently there are 3 description atoms visible in iTunes content.
desc = small description (255 chars) visible in iTunes. This description is also shown on the Apple TV v1.
ldes = long description visible at time of purchase from iTunes store with iTunes 10.2.x. Only visible in iTunes 10.2.x on assets procured from the iTunes store. If asset is removed from iTunes and subsequently re-imported this atom is not available to be viewed. On iTunes < 10.2.x this atom is always viewable. This description is also shown on the Apple TV v2.
sdes = store description. This description matches the description found on the iTunes store page for the relevant asset. Not viewable within iTunes. Does not appear to be of any use to end users at this time.