Etiquette is a set of practices and forms which are followed in a wide variety of situations; many people consider it to be a branch of decorum, or general social behavior. Each society has its own distinct etiquette, and various cultures within a society also have their own rules and social norms. Learning these codes of behavior can be very challenging for people who are new to a particular culture, and even old hands sometimes have a rough time.
The rules of etiquette govern how people behave. For example, the concept of greeting people politely and with respect is common to the codes of behavior of many cultures, although the way in which that respect is expressed may vary. In some Asian countries, for example, people may bow or clasp their hands together when greeting someone, while in the United States, people often shake hands, or hug each other in some subcultures.
It is safe to assume that there is an etiquette role for pretty much every situation one might encounter, from meeting the President of the United States to politely declining a meal in the Middle East. Because the social norms of different cultures are so different, many people study etiquette before traveling or entering a new social circle to ensure that they do not cause offense or embarrass themselves.
The consequences of lapses in etiquette may vary. At a minimum, someone might feel slightly awkward, and the people present at the time of the lapse might form an unfavorable impression of the offender. At worst, a lapse could cost a friendship or a job, and in some regions of the world, a serious breach could cost you your life. Depending on the culture you are in, people may be happy to answer questions about basic etiquette for you before you enter a potentially hazardous social situation, and people are usually amenable to apologies, especially when they are made promptly.
By treating people as respectfully as you know how and by using common sense, you will probably avoid most perilous behavior related situations. However, if you are planning to enter an unfamiliar culture, whether it be another country or the military, it pays to do research. It is especially important to review such guides if you plan to travel, as it is very easy to thoughtlessly cause offense, potentially generating a big social rift.