What are photoetched parts?
"Photo-etch" refers to a set of detail parts for a plastic model. They usually appear as a very thin sheet of brass, tin or aluminum with the parts all attached to a "tree," much like plastic parts. The level of detail available in these sets is amazing. Some kit manufacturers include photo etched parts in their kits, but the majority is available from smaller aftermarket companies as add-on parts.
More specifically, "photo-etch" refers to the method by which they are made. The sets are not cast in a mold like plastic, and they are not stamped out in a die cutter. The manufacturer draws a black and white template for the set and overlays the template on a blank, whole sheet of metal. The metal sheet with the template is set into a light-sensitive chemical bath, and exposed to a light source through the template. All of the white areas of the template allow light to pass through to the metal surface and "activates" the chemical bath so that it eats away the metal in those areas. By the time the metal is removed from the bath, the negative areas have been etched away by their exposure to the light.