U.S. Citizenship and Immigration Services (USCIS) is the government agency that oversees lawful immigration to the United States. USCIS employs 18,000 government employees and contractors who work at 250 offices across the world.
USCIS will likely play a role in the process of obtaining your green card. For example, the petition to request a family based or employment based immigrant visa will be filed with USCIS. Petitions for fiance visas are also filed with USCIS. If you are pursuing adjustment of status, you will have your interview at a USCIS office. After you get your green card, you will file your green card renewal requests with USCIS. Basically, as an immigrant to the United States, you will interact with USCIS at various stages in the immigration process.