Flat heads are countersunk (cone shaped under the head) with a flat top. Slotted drive has a simple slot for a flat bladed screw driver.
Truss heads are wide with a low profile rounded top. Combination drive can be used with either a flat bladed or Phillips screw driver.
Round heads are domed and look like half of a sphere. Slotted drive has a simple slot for a flat bladed screw driver.
Screws with machine threads for use with a nut or in a tapped hole.
Screws with an Allen wrench drive.