Body language is a type of nonverbal communication in which physical behavior, as opposed to words, are used to express or convey information. Such behavior includes facial expressions, body posture, gestures, eye movement, touch and the use of space. 
- Wikipedia

When we interact with others, we continuously give and receive wordless signals. Nonverbal behaviors include;

• the gestures we make
• the way we sit or stand
• how fast or how loud we talk
• how close we stand
• how much eye contact we make. 
• the quality, pitch, and speed of our voice
• ... and more

Key things to know

• Body language exists in both animals and humans
• It is largely unconscious
• Body language can't be (effectively) faked
• Body language messages exist independently of speech communication. Whether you're speaking or not, body language communication is still happening

Body language can easily reinforce, or contradict spoken communication. When mixed signals are present, the listener has to choose whether to believe your verbal or nonverbal messages. Invariably, they're going to choose the nonverbal because it's a natural, unconscious language that broadcasts your true feelings and intentions.

In relation to spoken communication, body language can relate in these ways;

• Repetition: they can repeat the message the person is making verbally.
• Contradiction: they can contradict a message the individual is trying to convey.
• Substitution: they can substitute for a verbal message. For example, a person's eyes can often convey a far more vivid message than words.
• Complementing: they may add to or complement a verbal message. A boss who pats a person on the back in addition to giving praise can increase the impact of the message.
• Accenting: they may accent or underline a verbal message. Pounding the table, for example, can underline a message.

Source: The Importance of Effective Communication, Edward G. Wertheim, Ph.D.

Types of body language

The many different types of nonverbal communication include;

• Facial expressions. The human face is extremely expressive, able to express countless emotions without saying a word. And unlike some forms of nonverbal communication, facial expressions are universal. The facial expressions for happiness, sadness, anger, surprise, fear, and disgust are the same across cultures.
• Body movements & posture. Consider how your perceptions of people are affected by the way they sit, walk, stand, or hold their head. The way you move and carry yourself communicates a wealth of information to the world. This type of nonverbal communication includes your posture, bearing, stance, and subtle movements.
• Gestures. Gestures are woven into the fabric of our daily lives. We wave, point, beckon, and use our hands when we’re arguing or speaking animatedly—expressing ourselves with gestures often without thinking. However, the meaning of gestures can be very different across cultures and regions, so it’s important to be careful to avoid misinterpretation.
• Eye contact. Since the visual sense is dominant for most people, eye contact is an especially important type of nonverbal communication. The way you look at someone can communicate many things, including interest, affection, hostility, or attraction. Eye contact is also important in maintaining the flow of conversation and for gauging the other person’s interest and response.
• Touch. We communicate a great deal through touch. Think about the messages given by the following: a weak handshake, a timid tap on the shoulder, a warm bear hug, a reassuring slap on the back, a patronizing pat on the head, or a controlling grip on the arm.
• Space. Have you ever felt uncomfortable during a conversation because the other person was standing too close and invading your space? We all have a need for physical space, although that need differs depending on the culture, the situation, and the closeness of the relationship. You can use physical space to communicate many different nonverbal messages, including signals of intimacy and affection, aggression or dominance.
• Voice. It’s not just what you say, it’s how you say it. When we speak, other people “read” our voices in addition to listening to our words. Things they pay attention to include your timing and pace, how loud you speak, your tone and inflection, and sounds that convey understanding, such as “ahh” and “uh-huh.” Think about how someone's tone of voice, for example, can indicate sarcasm, anger, affection, or confidence.

Message clusters

Body language comes in clusters of signals and postures, depending on the internal emotions and mental states.  Recognizing a whole cluster is thus far more reliable than trying to interpret individual elements.

• Aggressive Body Language: Showing physical threat.
• Assertive Body Language: Adult expression of desire.
• Attentive Body Language: Showing real interest.
• Bored Body Language: Just not being interested.
• Closed Body Language: Many reasons are closed.
• Confident Body Language: Appearing assured and comfortable.
• Deceptive Body Language: Seeking to cover up lying or other deception.
• Defensive Body Language: Protecting self from attack.
• Dominant Body Language: Dominating others.
• Emotional Body Language: Identifying feelings.
• Evaluating Body Language: Judging and deciding about something.
• Greeting Body Language: Meeting rituals.
• Open Body Language: Many reasons for being open.
• Power Body Language: Demonstrating one's power.
• Ready Body Language: Wanting to act and waiting for the trigger.
• Relaxed Body Language: Comfortable and unstressed.
• Romantic Body Language: Showing attraction to others.
• Sales Body Language: Using your body to sell.
• Submissive Body Language: Showing you are prepared to give in.
• Trustworthy Body Language: Showing you can be trusted.

Further References

ChangingMinds.org - Body Language

HelpGuide - Non-Verbal Communication

Wikipedia - Body Language

TEDx Body Language videos

Examples of body language