Dragons are the Barbaric Archipelago's (perhaps even Earth's) second dominant species, a race of powerful reptiles with two distinguishing characteristics: flight and fire generation, although not all dragons have both or either. A lot of dragons can be tamed and have human companions.
They are the main characters in the How to Train Your Dragon books.
Most dragons are four legged with a pair of wings and firebreathers. The fire from a dragon comes from two particularly small, almost undetectable holes in a dragon's throat. From these holes, it releases a breath heavy in oxygen and extremely flammable. This gives the dragon the ability to not only breath fire but to do it underwater (since the oxidizer component is present in the breath itself). A dragon may choose not to ignite its breath, and due to its oxygen rich nature, it can be inhaled by a human.
Dragons are also capable of living underwater as they have gills behind their horns that allow them to suck the oxygen in the water. Most dragons live on land, though. Their size and coloration varies depending on the species. The smallest dragons are the bug sized nanodragons with the biggest being the Seadragonus Giganticus Maximus. A dragon's gaze is said to be hypnotic, a staring contest with Toothless once made Hiccup very dizzy, and Toothless was just a baby. Their eyes also shine in the dark.
A dragon's diet consists mostly of meat, but they are supposed to eat plenty of vegetables and, weirdly, lots of wood, like small branches, twigs, and the bark of trees. This helps them with their firebreathing. A dragon that can't breathe fire gets very sick and eventually explodes.
Dragons have a few weaknesses. For one, they can be outsmarted by humans like Hiccup. Hiccup has done this many times, like with the Green Death and the Monstrous Strangulator. Many dragons can be controlled just by having a human yell at them. The biggest weakness perhaps is possibly the Dragon Jewel. The Dragon Jewel has the power to extinguish all dragons from the face of the earth, as inside the jewel, two dragons are present, one of which is suffering from a extremely deadly disease.
Behavior and Personality
Dragons in the books are described as being very sneaky selfish creatures who are never thankful for anything and will only obey under brute force and yelling. Most dragons pride themselves on this fact and will easily abandon their masters as soon as danger is on sight. Most dragons have a proud nature, often referring to themselves as aristocrats and green-blooded. Like cats, dragons have a cruel streak, and they like to play with their food before they kill it.
In Berk, many of them live as babies in the dragon nursery, a cave where they hibernate together. While hibernating a dragon enters into a very deep coma. Some dragons, however, don't hibernate and they are called 'evergreen' dragons. Dragons have three eyelids which they can open at will when sleeping. Only when all three are open the dragon is fully awake.
Dragons communicate with each other in a language known as Dragonese. This language is specially hard for humans to speak since humans don't have forked tongues. Dragonese is punctuated by shrill shrieks and popping noises, and sounds. The word "pishyou", when spoken by a human, for example, is pronounced very much like a sneeze.
They also love to play in fire since their skin is fairly fireproof, and some species are even completely immune to it.
Dragons' intelligence greatly varies in the books and it seems to depend on the species. Some dragons like the Monstrous Strangulator are shown to be stupid enough to be easily tricked into killing themselves, while others like the Seadragonus Giganticus Maximus are proud, intelligent creatures who love riddles and have been known to speak philosophically about abstract concepts like death.
Dragons start out as an egg, and after hatching they grow.
From Hiccup's prologue and epilogue in How to Train Your Dragon as his epilogue in How to Fight a Dragon's Fury, we learn that by the time he is old, most of the dragons have gone into permanent hiding in the northern waters.