Understanding body language is important to help you understand the full message that people are sending to you. With a few basic guidelines, you'll find yourself becoming much more skilled in reading people and understanding their body language.
Body language can be described as anything that communicates something to somebody else without the use of words. This isn't just gesture or universal sign language; body language is also shown through facial expressions, a voice, eye contact and many others. Anything that another person does and that you get a message from is considered body language.
For example, if you ask someone a question, they answer you and then look away quickly. It is an indication that they're probably lying, at least in part. This isn't one hundred percent correct, though; if they're just extremely nervous or shy they may very well have the same reaction.
There have been many books and studies on the subject of body language; however, in each case there are certain indications that mean the same thing. It is useful to learn what these are and practice paying attention to them while you're talking to people. You'll find that in a very short time you’ll notice some or all of these things on a fairly regular basis, and it will affect the way you deal with people in general.
Crossing the arms is a classic defensive gesture, and a pretty good sign that the person is not receptive to what you're saying, or perhaps even feels threatened by it. Continuous eye contact is a sign that the person is interested in what you're saying, while the shortage of eye contact usually means that the person is uncomfortable, bored or perhaps being dishonest.
It's important to remember that these are just guidelines, and none of them is likely to be true all the time. Understanding body language is an ongoing course that takes time, observation, and practice.