Something that's nonverbal doesn't use spoken words. Nonverbal communication might consist of hand gestures or facial expressions.
Use the adjective nonverbal when you talk about a method of communicating without spoken language, or a person who isn't able to use speech. The word verbal means "in the form of words," from the Latin verbum, "word," so anything that's nonverbal happens entirely without words. You can probably tell from a friend's nonverbal cues that she's uncomfortable — her face or body language might give her away.