Greetings and salutations
Human beings have evolved systems of greetings and salutations, modes of behaviour employed on first meeting someone, or upon meeting them again. Some of these are culturally-specific, some can be understood no matter where on earth a person is from; the latter are usually examples of non-verbal communication.
To smile and incline the head in a nod is universally understood. The handshake is a pervasive western greeting which has become known in many places, while bowing is known almost everywhere. Raising one's open right hand, usually with a slight wave, is also generally understood as a greeting.
Humans typically greet one another with an enquiry as to the other person's health or well-being, or a wish for the same; the most common forms are phrases that translate into English as "How are you?", "What is your news?" and "(Have a) Good Morning/Afternoon/Evening/Day/Night".
Form and formality
At various times, human societies have had elaborate rules for meeting and greeting, most of these built on age, social class and rank or status. As society has evolved and become more fluid socially, these rules have relaxed. In 2007, making a mistake in etiquette will no longer get one challenged to a duel, but errors in judgement can still irk and offend, so it is a good idea to know the basic social rules for the circles in which one will move. People tend to forgive greetings that are a little too formal for the circumstance more readily that those which demonstrate too much familiarity.