In 1969, the military draft was put in place to increase the number of young men who joined the military. The draft is a government system to legally require young people to join the armed forces. For a draft to be enacted, it has to be approved by Congress and the President.
The draft was implemented using a lottery system. Each lottery number, 1 to 365 (366 for a leap year), was randomly assigned a birthday. Men with the lowest numbers were called to join the military first. Younger men were typically drafted first- if you were 26 or older, you were not picked. There were exemptions for students enrolled in college or university. This led to higher rates of minorities and people with lower income being drafted, compared to wealthy, educated whites.
They have not used a draft lottery since 1975, but all males are still required to sign up for the Selective Service when they turn 18. Women are exempt from the draft.