Audio Description is an additional narrative that fits between dialogue, describing all significant aspects of what is contained visually. It describes who, where, what, when and why (if appropriate). Actions, facial expressions, clothing, scenery - anything important to conveying the plot of the story.
Most description is done by describing professionals, often authoring the script and then going on to record their voice as well. Where available, additional resources are sometimes used, such as the original script or a book telling the same story. For TV, video and DVD, the description has to be carefully balanced with the original soundtrack.
In theatre, there is often more than one describer, so as to share the load. This is pre-scripted, but actually described live so as to fit it with what is happening on stage.
Do you remember how the film starts in black and white and how Dorothy's house is whirled into the air during the cyclone? Whilst the house is spinning in the sky, Dorothy sees lots of characters sailing past the window. The house finally comes to a rest in the land over the rainbow and the film turns into full colour.
If you couldn't see the action on screen during the video you would have had no idea what was happening, there would have been bumps and cries and you would only have the music to give you clues. You also wouldn't know what Dorothy saw once she opened the door.
But the audio described version would have told you that Dorothy and Toto were dumbfounded as trees, a chicken coop and an old lady knitting whizzed past the window of the spinning house. The audio described version also tells you what happens once Dorothy opens the door after the house has landed.
"Dorothy peeps round the door as she opens it and she steps out into a world of colour. She gazes in wonder, as she steps out onto a yellow brick path, towards an ornamental bridge, across a bright blue lily pond, festooned on all sides by beds of fantastical, shiny multi-colourful hollyhock-like flowers, and in the background an emerald green meadow against a backdrop of pink, craggy rocks."
And you understand why Dorothy exclaims, "I don't think we're in Kansas anymore.".