What is the immigration policy in the United States?

Immigration to the United States is based upon the following principles: the reunification of families, admitting immigrants with skills that are valuable to the U. S. economy, protecting refugees, and promoting diversity.

That's a loaded question and cannot be simply answered. In general, the U. S. admits about 675,000 permanent immigrants annually. This is made up of refugees, family members of current U. S. citizens, workers who provide a skill needed in our economy, and the fulfillment of creating a diverse population. There are many subjective and continually changing elements to the policy, as well as complex components, depending on foreign policy at the time, relations with certain countries, etc. And then there are the temporary visas issued each year for tourists, temporary workers, students etc.