Chapman operates off two simple premises: 1. We all need to feel loved and 2. There are different ways to demonstrate love and we all have a preference for one.
Essentially, Chapman is saying that we all have a language for love. Just like my "bonjour" could be your "hello," I might hear love when you compliment my intelligence and you hear love when I give you a gift. It's a simple concept but one that most of us don't stop to consider.
The ramifications are huge. How many times has your partner, boss, child, parent, or friend been stunned to hear that you don't feel valued by them? They insist they've been working their guts out to show you how much they care. "How could you not see it?!" they say, indignant? You're shocked. How could they possible think that doing the dishes showed that they adored you? That's love languages at work. Because if they don't "speak your language" (demonstrate love in the way that makes your heart sing), their efforts, no matter how well intentioned, will fall short.
The book is an easy and quick read. Chapman also has a website (http://www.5lovelanguages.com/)where you can take the Love Languages quiz to discover which Language is yours. There's even a quiz tailored to kids. I completed it with my children and they loved it.
A final note: I have heard this book referenced in the context of faith several times, but it is by no means limited to people of any particular faith or faith at all.