Where one being wraps their arms around the body of another being and holds them close for a moment. The psycho-physical message is along the lines of: "It's okay, I've got you"
Is mostly non-sexual, unless you go for a grope halfway through it ;)
Not strictly limited to humans, hugging can be done by other species, replacing arms with wings, tentacles, hooves, etc....
Within friendship, hugging can be a great way of expressing support and platonic love :)