The term dog kennel can refer to either an indoor shelter (such as a crate) or an outdoor shelter (such as a doghouse). Generally indoor kennels are small and made of wire or plastic so as to be light and easy to move, while outdoor kennels are made to stay in one place and may be simply a fenced area with a roof.
Choosing a kennel depends largely on what type of dog you have. The kennel should be large enough that your pet has room to move around in; if he can’t turn around easily, the kennel is much too small. (If you’re buying the kennel for a puppy, you should naturally get one based on the size he’ll be when fully grown). There should be enough room so that if the dog has an accident, he doesn’t have to lie in his own mess. Outdoor kennels will generally be larger, and may have enough space for the dog to run.
Dogs can be big chewers, so you want to ensure that it isn’t possible for your dog to chew a hole in his enclosure and escape. Commercial kennels should be designed with this in mind, but it’s still worthwhile to check occasionally and make sure that no holes have appeared.
Most important, of course, is the health, safety, and comport of your pet. If he’s to be left outdoors, there needs to be cover from the elements; dogs should never be left without a way to get out of the sun and rain! This could be a roof or an igloo dog house with a flap to keep the weather out. Obviously, the more time your pet will be spending in his enclosure, the more elaborate and comfortable it should be.