What is this and is it dangerous?
That's a wheel bug (Arilus cristatus), a species of assassin bug named for the wheel-shaped projection on their thorax. They tend to prey on pest insects, but they do "bite" humans with their proboscis. The result is an extremely painful, itchy welt, though non-venomous.

These one's don't attack unprovoked, however, so generally just when trying to handle them. You should handle them with gloves.