Low Carbon Steel Carriage: Round head bolt with a square neck under the head, and a unified thread pitch. 
Made from low or medium carbon steel.
Hot-Dip Galvanized Steel Carriage: Carriage bolt made from low or medium carbon steel with a galvanic zinc finish applied.

Low Carbon Steel Carriage: The square neck is designed to keep the bolt from turning as a nut is tightened.
Hot-Dip Galvanized Steel Carriage: Same design advantages as a low carbon carriage bolt but with a thicker protective coating for outdoor use.  Often used in outdoor furniture.