Layouts represent the whole of your MIDI Designer layout: all banks, all pages, and all controls. Layouts are files that have the extension "mididesigner" so an example would be, "Cannes May 2012 05.mididesigner". MIDI Designer adds the file extension for you.
Layouts may be saved, loaded, and emailed. Find this functionality under More -> Config -> Actions.
Layouts may also be opened from email. Follow the default instructions in the email which contains the layout.
In addition, you can drag and drop layouts to and from MIDI Designer via iTunes.
Note: layouts contain your presets, so if you open your layout on another iPad, you will find your presets are also available.