A resident alien is an individual who is a non-U.S. citizen but lives in the United States. This status is given to those who are foreign-born, have a green card, or have passed the substantial presence test.

A green card, or the more formal term "Legal Permanent Resident Card", makes an immigrant eligible to live and work in the United States. To obtain one, they may have to prove several requirements, including having a job, having financial solvency, and more.

The substantial presence test is required for those immigrants who have not obtained a green card but have stayed in the United States for more than 183 days. Under this test, those immigrants must demonstrate that they are making an active effort to make the United States their primary residence. This includes acquiring a job and a residence, having family ties, and other aspects of settling into the country.

Once a resident alien is established, they will be subject to the same taxes as U.S. citizens. The Internal Revenue Service requires resident aliens to report and pay taxes on any income generated from U.S. sources. This includes wages, dividends, investments, and many other types of income. Additionally, resident aliens generally face the same social security and medicare taxes as U.S. citizens regardless of the country of their citizenship.

Overall, becoming a resident alien provides an avenue for non-U.S. citizens to live, work, and pay taxes in the United States. By having the legal status of a resident alien, they are able to make the United States their primary residence, pay the same taxes as U.S. citizens, and gain other benefits such as exemptions from luxury or other taxes. The process of becoming a resident alien is difficult and requires satisfying numerous requirements, but for those that are successful it can be very rewarding.