This is a pretty interesting book. It's all about how little changes result in large effects. He starts off talking about fashion trends, crime rates, and other behavioral epidemics. There are many epidemics that take place after a particular inflection point. These epidemics aren't disease-related, but they are about how various ideas and memes that spread from person to person.
There are three rules for an epidemic to take off.
Rule #1 is the Law of the Few. This law is about how ideas spread through the influence of a few people, and how a there are three types of people who do so. They are:
Connectors: People who know many people. They are comfortable with "weak link" relationships, e.g. sending Christmas cards to acquaintances, and maintaining such relationships. Think about your current friends, you probably met many of those people through just a few people. Those are connectors.
Mavens: People who absorb and disseminate information. Find the latest cool tip/trick to save money? Need someone to tell you what's the best vacuum cleaner to buy? Who would you turn to? That's the maven.
Salespeople: This is a person who can sell you on things. The salesperson, if enthusiastic about a particular product/idea, can infect other people with the idea.
If you have an idea you want to spread, pass it along to these three people. But it's more than just knowing the right people.
Rule #2 is the Stickiness Factor. The message that your trying to spread must _stick_ in the mind of people who listen to it. If it's a commercial, it must be entertaining. If it's a new product, it must be good. This part is pretty obvious.
Rule #3 is the Power of Context. An idea will only take off in a particular environment. There are interesting examples of this in the book, which aren't really that obvious.
I recommend this book. I don't think it's as life-changing as some people make it sound, but it does provide some food for thought. The author takes these simple ideas and beats you over the head with examples, so at least they stick :).