CDX is the most common exterior plywood grade. It comes in 3/4-, 5/8- and 1/2-inch thicknesses. Appearance stamps on plywood range from cabinet-grade (A) to construction grade (C and D). Exterior plywood carries an X stamp, which identifies it as suitable for use outdoors. Beside CDX, you can also find better grades, such as ABX, ACX and even BCX.
Marine grade plywood is similar to standard exterior-grade plywood, but its core consists of knot-free sheets, eliminating the possibility of the formation of water pockets within the structure.
Pressure-treated plywood usually has a greenish hue and a slightly oily surface. Manufacturers produce it by infusing regular plywood with chemicals under pressure to give it added resistance to mold and mildew. Treatment chemicals include chromated copper arsenate (CCA), ammoniacal copper quat (ACQ), copper azole (CA) and -- safest of all -- borates.
Oriented strand board (OSB) is an engineered plywood substitute that many builders use for sheathing and roof decking. It's constructed with waterproof glue and performs best when only semi-exposed to the elements, as it is when covered with siding or roofing. It's less expensive than plywood and available virtually everywhere.