An NGO, or non-governmental organization, is a nonprofit organization that works independent of government to address social issues. This may include research, but NGOs' primary focus is on taking action on the ground. NGOs typically have a broad national or international scope to their work, whereas nonprofits are typically more locally focused.
The term "think tank" refers to a variety of nonprofit research organizations that produce public policy research, analysis, and often conduct advocacy, as well. Some think tanks are nonpartisan, and some are affiliated with a political party or ideology. It's important to understand the point of view of the think tank whose reports you are using so that you can place them in context for your audience.