There are tons of different types of plywood and the selection can be overwhelming.
STRUCTURAL PLYWOOD
Structural plywood is used for roof sheathing or subfloors, and will probably be covered up with something more decorative. Its primary goal is to be as strong as possible. Structural plywood comes in both C and D grades, but no higher.
It will be less expensive than other options, because it's a softwood plywood that has knots and imperfections. You can use it for furniture making, but will take a lot more work to make it look nice.
LUMBER CORE PLYWOOD
All those thin veneer layers take a lot of effort to glue together, which can increase the cost. For thicker decorative plywood without the huge price tag, you may want to consider lumber core plywood.
This type of plywood has a thick core of solid wood, sandwiched between thin wood veneers like traditional plywood. The core is typically made of an inexpensive softwood like Douglas fir, with ornamental wood on the outside layers.
SANDED PLYWOOD
You may see the term "sanded plywood" when shopping for plywood sheets for your project. These are typically softwood plywood that has been sanded smooth at the factory, which can save you a step when building furniture.
Softwood plywood or sanded plywood is usually cheaper, and is made out of pine, fir or spruce. It's more likely to dent or scratch, so it shouldn't be used on tabletops or desks that get a lot of heavy use.
FURNITURE OR CABINET GRADE PLYWOOD
Cabinet grade is typically hardwood plywood, and is sometimes called decorative plywood because the outer layers are a more attractive wood species like maple or walnut.
Hardwood plywood may have a different species for the inner core, or even a solid wood core, which can save you money. Both types can be found in grades A-D.
EXTERIOR PLYWOOD
Exterior plywood is specifically made to withstand the elements. The veneers are held together with water resistant glue, so the layers don't peel apart. It's typically used as exterior sheathing plywood on the outside of a house, such as roof decking.
Also, it usually has a higher number of veneer layers than interior plywood. This is so that there is more glue in the plywood, which makes the sheet more resistant to water and mildew. However, you still need to apply a water resistant coating, like exterior paint, to protect the wood from rotting.
MARINE PLYWOOD
Marine plywood is one of the highest grades out there (aircraft plywood is even higher - no pun intended!) Every knot hole in each layer of veneer is filled, and water resistant exterior glue is used so the layers won't separate. This is why I recommend waterproof wood glue on all my outdoor furniture projects as well!
Not only does marine plywood use weather resistant adhesive, but it can also be made with rot resistant wood. Tropical plywood can be used for outdoor furniture because of its weather resistant properties.
