What is an NGO?

As you've probably observed, this Blog is about NGO security. But what exactly is an NGO? Coming up with a good working definition is actually a little more complicated than you might think. A good reference is Peter Willetts' "What is a Non-Governmental Organization?" that appears in the UNESCO Encyclopaedia of Life Support Systems. Even if you're in the humanitarian field and think you know what an NGO is, this paper is a good read. (Thanks to Alexandre for the tip.)