DOM (Drawn Over Mandrel) has a more consistent size and shape so that it has consistent properties in any direction for lack of a better term. DOM still does have a seam that is fused therefore you still have a heat affected zone down the length of the tube. After the tube is formed and welded, it is then pulled over a mandrel to accurately form the shape which provides a more uniform wall thickness, ID, OD. This also does some cold working and can possibly add some strength.
CDS (Cold Drawn Seamless) tubing is formed from a solid carbon steel bar by drawing it over a mandrel to form it's shape. Therefore there is no welded seam. This process also cold works the steel and improves it's mechanical properties.
Chromoly Tube is basically CDS tubing made from 4130 alloy steel. Standard CDS tube is typically 1018 or 1020 carbon steel. The 4130 alloy has greater tensile and yield strength than 1020 while maintaining good ductility. However it does require certain things be done before during and after the welding process to ensure the weld integrity as well as the quality of the base metal in the heat affected zone of the weld.