Ressurect means bringing something back to life and therefore it's alive. Undead is its own category and normally means the being is raised and reanimated but without a soul so therefore being undead. You can be alive and undead. So all dragons are not undead as incorrectly stated by Ketick1998.
the main questline says that dragons have returned to skyrim by alduin,who ressurected them back to life.
however,spells such as sun fire won't deal any damage to dragons (idk about the skeletal dragon)
Skeletal Dragons are Undead because their bones were animated via magic, and share the same general origins as regular skeleton monsters. Yes it can use shouts. It is a dragon, a direct offspring of the gods after all, and shouting is intrinsic to its existence. But it's incredibly weak compared to other dragons, and its undeath is proven for certain by the fact that you don't absorb a soul from it when it is killed. It doesn't have a soul to absorb. (can you soul trap it to a gem though?)
Regular dragons aren't undead because Alduin's method of resurrection isn't the same as reanimating a corpse. He travels to Solsteim, directly consumes and manipulates souls, and has dominion over the dead. By returning the soul of a dead dragon to its bones, the dragon's very identity as an immortal being created by the gods is resurrected, and it becomes an example of "I Think Therefore I Am" in the mightiest sense. This is why dragons burn only when the Dragonborn kills them (town guards, giants, etc. tend to leave corpses that can still be looted but do remain whole)-- because their form is eternal, and only the Dragonborn dealing the final blow and absorbing their soul can take a dragon out of this world. Its flesh, its form, its very identity as a dragon, are all tied to its soul. Skeletal dragons don't have souls. They are just dead bones reanimated with typical conjuration magic, with mild shouting abilities left over from its residual power as a god.
The quote from Descartes is further supported by examples of humans becoming gods like Talos and the New Sheogorath, as well as the rare instance of a character achieving Chim. Heck, all of existence including the gods themselves are nothing but the expression of ideas brought to life by chunks of imagination torn off the embodiment of raw creation. Dig through the lore, you'll figure it out.