Where another screw would have a straight line or a cross shape, Robertson screws have a square socket. Also known as square or Scrulox screws, they're designed to make it easy to keep a square drill or screwdriver head in place while fastening. This makes them a little easier to work with one-handed.
Invented by Canadian P. L. Robertson in 1908, Robertson screws are still more popular in that country to this day, though their use has expanded around the world, primarily for woodworking, construction, and assembly of machinery like breaker terminals. The slight taper deployed by Robertson was an improvement on an earlier square screw design patented by American Allan Cummings in 1875, and made it easier to "cold form," which facilitated mass commercial production.