Well, first of all, NGO is short for non-governmental organization, and basically what this entails is an organization formed by citizens, rather than the government, to perform some sort of social service: helping with education, supplying food and nutritional needs, providing medical assistance, saving environmental elements, building homes or schools, creating after-school programs, and many other imaginative and well-meaning activities. NGOs exist throughout the world, from Manhattan to Timbuktu (literally), and offer great opportunities to help others, animals, trees, or whatever you like.
Many NGOs are founded in wealthier western countries and rooted in impoverished countries, and others are founded within their host countries. NGOs can be massive with branches and pie-crusted fingers all over the world, such as Oxfam or Green Peace, or they can be tiny operations with literally a couple of teachers out in the jungle providing English classes to kids from a remote Mayan village. In other words, there are several incarnations of the standard NGO, such as BINGOs (big international NGOs) like the Red Cross, RINGOs (religious international NGOs), and even the paradoxical GONGOs (government-operated non-governmental organizations?).
Whatever the case, for the purposes of this site, I will be presenting smaller NGOs, localized in their focus and generally located in more backpacker-y spots like Central and South America, Southeast Asia, and Eastern Europe. The idea behind this is that these NGOs in general have less publicity than the big boys, receive less support via volunteer and funding, and usually offer volunteer opportunities with much less bureaucracy to wade through. Small and grassroots NGOs can usually gain some true benefits from international volunteers willing to devote part of their travels to pitching in on something good.