Many dogs with long or double coats are suspectible to "matting" of their fur. Matting is tangled clumps of fur that usually happens if your dog is not brushed or groomed regularly. Brushing or combing matts out can be uncomfortable for you dog, not to mention time consuming and difficult for owners.
Dogs with a dense double coat can have matting to their undercoat that goes unnoticed. Regular brushing and grooming is the only way to maintain a matt-free coat, however, there are some excellent shampoos and conditioners that can help make brushing out matts easier as well as preventing further tangling.