These are aggressive. They sting multiple times. In my experience they go straight for the head when disturbed.
They typically make a bulbous nest in shrubs or trees(or on the side of structures). When the nest is build on shrubbery or a tree it is intertwined with the branches, so if you touch the branches it will send a signal to the nest for them to come out and protect it(run away!).
If by chance you are going to treat this nest yourself be careful! I suggest that you do not do this with out wearing protection.
If you treat the nest during the day do not remove the nest after treatment, returning workers from the field will return find it missing and will rebuild the nest quite quickly.
Here you can read about Bald Faced Hornets
Here are images of Bald Faced Hornets
Here are some images of some nests.