The Selective Service System is an independent agency of the US government that maintains information on those potentially subject to military conscription. Under current law, all male US citizens between 18–26 years of age are required to register within 30 days of their 18th birthday. In addition, non-US citizen men between the ages of 18 and 26 (inclusive) living in the United States must register. This includes permanent residents, refugees, asylum seekers, and illegal aliens. Currently, citizens who are 17 and 3 months old can pre-register so when they turn 18 their information is automatically added into the system. Women are not required to register.
Registration with Selective Service is also required for various federal programs and benefits, including naturalization, student loans, job training, federal employment, etc. Failure to register as required is grounds for denying a petition for US citizenship.
So if you are a man between the ages of 18 and 26, you must register for the Selective Service and provide proof of your registration to USCIS. If you are 26 or older but under the age of 31, you must provide proof that you registered with the Selective Service when you were required to do so. If you were required to register and did not, you must bring to your interview both a written statement explaining why you did not register and a letter from the Selective Service System indicating your status.
Registering does not mean you are joining the military. The U.S. does not have a drafted military; the decision to join the military is entirely voluntary. Deliberately avoiding registration or even not knowing to register can create a significant problem for a male permanent resident that wants to naturalize as a U.S. citizen. To solve this problem you can easily register for Selective Service by visiting www.sss.gov. And if you are not sure if you have registered write an instant Official Letter of Verification to get proof.