Developer is a powder usually made up of iron fragments. In a copier or laser printer, developer is used in conjunction with toner to print an image.
Developer is made of magnetic ferrite or tiny fragments of iron. These materials are chosen because they are conductive, and the copier process works by using static electricity.
Copiers communicate where the ink needs to go using static electricity and photo-conductivity. The copier creates an image using static electricity. It then spreads developer on the paper, and the developer only sticks to the parts that are charged with static.
The areas of the page coated with developer have an electrical charge. The toner sticks to these areas, creating a printed image. Heat is used to permanently bond the toner to the paper.