These crates have wire doors and vents on both sides for proper air circulation. The smaller plastic crates come with handles so they can be easily moved. The larger crates need either wheels or a crate dolly to be moved. The reason for this is that the assembled crate with a dog inside will be too heavy for most people to pick up by a handle. Check with your airline if you are buying this crate for airline travel only.
Some airlines have different restrictions.
Wire crates are used mainly for home, kennel and shows. These crates are made completely of wire - all four sides, top and bottom. A pan is used in the bottom of the crate for the dog's comfort as well as for easy cleaning. This pan is either galvanized metal or heavy plastic, depending on the manufacturer. They come with handles to make carrying easier.