Influence explains various ways and techniques other people may use to influence you so that you do something they want you to do. This book also explains how to defend ourselves from such exploitive influences. Of course if you know the techniques you can use them to influence other people.
The basic idea presented in this book that our lives are often governed by fixed-action patterns ("click-whirr behavior", as they're often called in this book.) However other people to influence us and take advantage of us can use those fixed-action patterns. The book explains different kinds of fixed-action patterns, how they work, how they are used against us and what can we do to protect ourselves from being abused. The following are the techniques that the book discuses.
Reciprocation We feel obligated to return favors. People might take advantage of it by asking favors much bigger than those they've given us.
An example is the Krishna people giving flowers as "gifts" to make us give them money.
Commitment and consistency If we commit to something we are more likely to stick with it because we want to maintain consistency. An example is people who place a bet on a football game are more confident of their teams chances of winning.
Social proof We do what other people do. It's usually advantageous to just follow other people instead of trying to figure out what the correct behavior is. We can be manipulated by fake social proof. Some examples are canned laughter in TV shows or bartenders filling up the tip jar.
Liking People whom we like easily influence us. Some people will try to take advantage of this by making us like them and then asking favors. Also, research shows that we link beauty with talent, kindness, honesty...