Formal and Informal Greetings
When greeting people you have a choice between formal and informal greetings. Formal greetings are those you use with adults and individuals with a title (like: Dr., Mr., Sgt.). Informal greetings are those you use daily with your friends. An example of a formal greeting in English is: "Good Morning." An example of an informal greeting is "Sup?" While it is okay to use a formal greeting with your friends, it is considered very bad manners to use an informal greeting with an adult (unless it's a relative like your mother or step-father).

In the textbook
In general, people who speak Spanish tend to be more tactile (meaning they touch) than other cultures. Looking at page 2, you see that in every picture the people are touching each other. There is nothing wrong with those people and this is just a cultural difference between how we greet and how others do.

Buenas noches is used as a greeting in our text. It can also be used as a farewell. Often times we say good-night to each other as we leave, hang up the phone, etc. So how do you know the difference if it's a greeting or good-bye? The context of the situation will tell you. If people are walking away from each other and say "Buenas noches," you know they are giving a farewell.