Truss head and pan head are both types of screws with different head shapes.
A truss head screw has a low, rounded head with a large bearing surface. It is similar to a countersunk head, but the head is not tapered. Truss head screws are commonly used in applications where a flush or low profile is desired, as the large bearing surface of the head helps distribute the load and prevent the screw from sinking into the material.
A pan head screw has a rounded head with a slightly domed top and a flat bearing surface. It is similar in appearance to a small, shallow dish. Pan head screws are commonly used in applications where the head needs to sit above the surface of the material, as the flat bearing surface allows the screw to sit securely without sinking in.
In summary, a truss head screw has a low, rounded head with a large bearing surface, while a pan head screw has a rounded head with a slightly domed top and a flat bearing surface.