Feathers are user-installed add-ons that change Nightingale's visual appearance (yes, like skins & themes, only we're birds - so we call them Feathers). Feathers can be as simple as mere CSS & image changes, or as complex as full blown layout and UI changes incorporating XUL, HTML, CSS,
Flash, and SVG.
There are two parts to a feather: the layout, and the skin. The layout is the skeleton which determines where elements lie and what elements are present. The skin determines what images, colors, and fonts are used.
Most feathers reuse existing layouts and display them in new ways, like iBird, Ububird, and so on. Some feathers only provide new layouts for existing skins, such as mig's Jump To feather for Rubberducky. Finally, a feather can combine a skin and a layout to provide a totally customized experience as komi's awesome Sidecar or Pino's crazy-cool Birdie.
The best way to get started making feathers is to either copy an existing feather and change it, or to use the Feathers Wizard which comes as part of the Developer Tools.