What is an ONG/NGO?

NGO Monitor defines NGOs as autonomous non-profit and non-party/politically-unaffiliated organizations that advance a particular cause or set of causes in the public interest. The range of causes on which an ONG can focus is unlimited, but a cardinal principle is that NGOs operate in a manner consistent with the objectives for which they receive funds. Donations are an ONG´s lifeline because they are independent organizations. Funding can come from governments, the UN, private trusts and philanthropies, individual donations, religious institutions, and, in many cases, other NGOs. NGOs can contribute to democracy through challenging governments and promoting social interests, but they themselves are not democratic institutions and have no democratic accountability. An NGO is only accountable to its particular funding organizations and members.

History of ONG

There have been around since the nineteenth century. One of the oldest is the Red Cross. The formal recognition of NGOs is from Article 71 of the Charter of the United Nations (1945):
The Economic and Social Council of the United Nations may make suitable arrangements for consultation with non-governmental organizations concerned with questions of competence.
 
Besides participating in the United Nations system, so does the level of national states corresponding observers, consultants, project performers, as a form of social pressure citizen, etc..
 
Since then the Economic and Social Council has gone from 41 NGOs in consultative status with in 1946 to about 2350 NGOs (2003). Many more if you include those only locally and regionally.