The book covers primarily how we are "influenced" into making certain decisions by certain triggers. Cialdini gives plenty of examples and situations to be aware of and how marketers and salespeople use these pressure points to bring you over to their point of view or sell you stuff you don't really need.
It's a very interesting book, and I often found myself identifying with many situations that are highligted in the book. You know those situations where you are left with something you have just purchased and have no idea why you actually purchased that thing in the first place? The book outlines why you did what you did.
Some of the concepts he covers included Social proof. Where our actions are dictated by what everyone else is doing, even though doing what everybody else is doing might seem bizarre to an outsider to the situation.
He also writes about how salespeople work hard to get us to believe that they are on our side, and leave you feeling remorseful if you don't buy a car from this nice guy that is truly "on your side".
There is an interesting chapter about how we often surrender all logic when dictated to by a person of seeming authority. How people will do things they know to be intrinsically wrong, but, because they are told to do it by a person in authority they will comply even though they are visually anguished to be doing so.
It's a great book, and one that you will no doubt find interesting even if you're not a psychology major or marketing person.