NGOS and Think Tanks are very similar and are often colloquially used relatively interchangeably. Think tanks are institutions affiliated with universities, foundations, advocacy groups, and other organizations that generate policy research. Some are ideological, and others are non-partisan. NGO stands for Non-Governmental Organization and is used to refer to a wide range of nonprofit organization that operate independently of any government, typically with the purpose of addressing a specific social or political issue. This term includes many Think Tanks but also includes organizations like Amnesty International or the American Red Cross.
Many NGOs and Think Tanks do not require their interns or employees be U.S. citizens. These positions can also often be a bit more flexible than government jobs, offering more remote/hybrid options and more internship term variety.