Below are some difference between Karcher WD1 Classic, WD3, WD4, WD5 P and WD6 P S. If you are deciding between them, the information may be helpful to you.

• Container Material
  Only the WD 6 P S uses a stainless steel container, the other models use a plastic container.
• Container Capacity
  The WD1 Classic has the smallest container which holds up to 4 gallons whereas the WD3 holds up to 4.5 gallons, WD4 capacity is 5.3 gallons, WD5 P capacity is 6.6 gallons, and the WD6 P S has the largest container which holds up to 7.9 gallons.
• Removable Handle
  The WD3, WD4, WD5 P and WD 6 P S have a Removable Handle that can be mounted to the hose so you can connect it to the accessories, or be detached from the hose so you can connect the accessories to the hose directly. The WD1 Classic does not have such design.
• Blower Function
  The WD4 has no blower function whereas the others have the blower function to help move dirt and debris from hard-to-reach areas or where vacuuming is not possible.
• Power Tool Outlet and Adapter
  The WD5 P and WD6 P S come with a Power Tool Outlet and Adapter that allow you to connect power tools such as saws and sanders. Whereas the WD1 Classic, WD3 and WD4 do not have this outlet and adapter.
• Hose Length
  Karcher WD1 Classic has the shortest hose which is 5.9-foot in length, whereas the WD3 hose is 6.7-foot long, the WD4 has a 7.5 foot hose, the WD5 P and WD 6 P S have the longest hose which is 11.5-foot in length.
• Filter and Filter Bags
  All of them have a fleece filter bag for dry vacuum. The WD1 Classic also uses a non-woven filter bag for dry vacuum cleaning. When doing wet vacuum cleaning, this non-woven filter bag should be removed, and a foam filter should be installed. The WD3 uses a round cartridge filter for wet and dry vacuuming without additional filter replacement, similarly, the WD4, WD5 P and WD 6 P S use a Flat Pleated Cartridge Filter.
• Semi-Automatic Filter Cleaning System
  The WD5 P and WD 6 P S feature a Semi-Automatic Filter Cleaning System which utilize pulsed air to move dirt from the filter to the container at the push of a button, thus save time on cleaning filter, help extend filter life and restore suction power quickly. Whereas the others do not have this feature.
• Wheels
  The WD1 Classic, WD3 and WD4 have 4 casters whereas the WD5 P and WD6 P S have 5 casters.
• Cord Length
  The WD1 Classic has the shortest cord which is 16-foot in length, the WD3 has a 20-foot cord whereas the WD4, WD5 P, WD6 P S have a 26.25-foot cord.
• Weight
  The WD1 Classic is the lightest whereas the WD6 P S is the heaviest which weighs 21.8 lbs without accessories.

For more difference, read Karcher WD1 Classic, WD3, WD4, WD5 P and WD 6 P S comparison table below:

Some similarities:
All are designed for wet dry vacuum cleaning.
Come with a floor nozzle, crevice tool, hose, and 2 tubes.
Hose diameter is 1.38 inches/35mm.
Carry a 2-year limited warranty.