I'm not sure about the exact names, but there are several closely related oxides of iron, both red and black are oxides. The red stuff is the "rust" that forms on the inside of your gas tank, as it eats holes in the steel. It's hard to get off the steel, promotes more rust, and doesn't support paint very well. The black stuff is typically what the various rust converters make out of the red rust. It can be fairly easily washed or brushed off, and once removed, whatever remains provides good adhesion for primer or paint.
Maybe someone can speak with more detail?