EDM is a high precision process of removing material via blasts of electrical current. It is capable of achieving dimensional accuracy within ±.0001". Funny enough, the vehicle by which material is removed at the atomic level is not well understood.
An electrode (e.g. carbon) is fabricated to the desired final shape. It is then plunged into the workpiece in a series of cycles under high current.
An electrified wire is used to cut metal like wire is on cheese. Very intricate parts can be created with out the need for a custom tool.
EDM is a common, yet expensive process sometimes necessary in mold making. Some geometries cannot be created by traditional machining, so EDM is employed.