CDX Plywood - What Does the "X" Really Mean?
The letter "X" in CDX refers to the type of glue used in the factory to bond the plywood veneers. Some mistakenly think the "X" stands for "exterior"—which is not true, exactly. The letter stands for exposure, which means the plywood is tough enough to withstand a little moisture—but for only a short time.
For example, let's say you're in the middle of building a garage, and you've just finished putting up CDX plywood as the sheathing for the walls, which will eventually be covered with siding. But now you've discovered that your siding won't be delivered till the end of the week. Can you leave your CDX plywood exposed for a couple days? According to grading standards for CDX plywood - you can leave it exposed temporarily without worrying about water damage. In fact this is one reason why CDX plywood is used so extensively in outdoor construction. The glues and adhesives are made to be water resistant (long enough for to wait for a late delivery), but they're not strong enough to withstand rain and snow indefinitely.
CDX plywood is used under shingles and roofing felt, on walls (just behind the siding and insulation), and as a sub floor (just under the carpet pad or a tiled floor's durock). Although it's easy to find at your local home center, it's probably not the best choice for smaller projects, like a bookshelf or other furniture around the house. However, I've used it for creating rough shelving in a basement or garage, and it works fairly well for those kinds of projects where function is more important than appearance.
The letters "C" and "D" represent the quality of the surfaces—front and back. If we consider that the letter "A" is the best level of plywood veneer you can buy (that means plywood without any noticeable knots, holes, or repaired defects), then letters C and D certainly indicate a lower grade of quality when talking about plywood in general—and the price will reflect this.
CDX plywood is fairly easy to spot on the shelf. You'll see plenty of football-shaped patches and shallow holes and indentations on both sides. Considering what CDX plywood is normally used for (see below) it's not going to matter really what the panel looks like - as long as it is in one piece.
Below is an example of what you can typically expect to pay for different types of construction-grade plywood panels. These prices can change over time, and differ from one store to the next. The price chart is an example only.