Selective Service and citizenship applies for men who are between the ages of 18 and 26. Men in this age demographic must register for Selective Service. Although there is no military draft anymore in the United States (as of January 1973), the United States still keeps track of eligible men who can fight on behalf of the United States in case of a national defense emergency.
The Selective Service System is a list of eligible men who can be called into a military draft under 50 U.S. Code Ch. 49. When registering for Selective Service, you are not committing to join the military. You are registering to commit to the United States armed forces in the event that a massive emergency or war breaks out.