NGOs and global politics

Any examination of NGOs must commence with a discussion of definitions. 'NGO' is an abbreviation for 'Non-Governmental Organisation', that much is clear; but what does such a term include, and what does it not cover? Does it apply to organisations active within nation-states or just to international NGOs (INGOs)? How does it differ from other terms such as 'Non-State Actor' (NSA) or 'Civil Society Organisation' (CSO)?