Dragons are giant lizard like creatures - by comparing concept art we can assume an adult male dragon is around 5 meters from head to claw. The dragons traditionally walk on all four and sport features (frills, spikes, fins) and colors that are related to their elements.
The true form of a female dragon is unknown, as the only known female dragon in the games is Cynder, who is a teen, and her 'adult' form is caused by a corruption ritual, and one can assume that form is unnatural as it is repeatedly called monstrous.
Dragons can theoretically sport all colors, and have eye colors corresponding to their element. Orange for Fire, yellow for Electricity, blue for Ice and green for Earth. They are known for a fact to be able to wield any one of these elements, while it is unknown if anyone besides Cynder and Malefor can breathe Shadow, Wind, Poison, or Fear.
The Purple Dragon is born once every ten generations, a special creature with extraordinary powers that can wield all the known elements. The only known purple dragons are Spyro and Malefor, however the latter insists there have been many.
Ignitus has stated that after a dragon has died, they bind themselves to nature and offer hope for the future.
Dragons have for centuries been revered by other species, enough that the moles built a city in honor of their friendship with them. When Malefor appeared however, dragons quickly became less adored.
After failing to protect the dragon eggs, the dragons went to battle against the Dark Army. It is unknown if all but the Guardians were killed in those battles. Besides the main cast of characters, dragons don't appear again before Dawn of the Dragon.