A think tank is typically a public policy institute, dedicated to finding facts about, and thereby advancing, a particular theory or policy. An example is the Cato Institute, a libertarian institution which occasionally contributes positively to conservative viewpoints.
The Heritage Foundation is a pro-free trade think tank located near Union Station in D.C., which raises substantial donations from China. In 2016, Heritage promoted some pro-abortion nominees for Donald Trump to choose from in picking someone for the U.S. Supreme Court.
The Federalist Society is a think tank that has repeatedly given advice to Republican presidents and has prevented pro-life judicial nominees from being selected for the U.S. Supreme Court and elsewhere on the federal bench.