Hornets don't tend to be as aggressive as one may think - they tend to only attack when they feel threatened, like when someone comes within a threatening distance to their nest - this is usually 2-3m.
Hornets are actually a variant of wasps that live in large, highly organized, colonies. The main difference between wasps and hornets is that hornets are larger in size, though not aggression.
Hornets tend to build their nests in high areas, though have been known to build underground. Hornet nests which are elevated are sometimes free-hanging, using a stalk called a petiole which attaches to the nearest object such as a tree branch.