Each type of dragon in Rise of Berk have a special icon that tells of what specialty it possesses within battling. These are split into the three groups- Health, Damage and Range.
Health Dragons specialize in taking a lot of hits. They have low attack damage and must be close to attack. They are good if you want them to take hits so other dragons don't have too.
Damage Dragons specialize in dealing heavy damage to ships, they have low health and must go somewhat close to attack. They are good to quickly deal damage and quickly destroy enemy ships.
Range Dragons dragons specialize in attacking ships from far away. They deal moderate damage but have very low health. They are good to slowly deal damage without taking many hits.