During the Vietnam War, the US Army was made up partly of volunteers, but also partly of soldiers who had been drafted. Overall, the Army was skewed in its demographics. It was made up of a disproportionate number of young men from poor and working class backgrounds.
One reason for this was the draft. About one-third of the entire Army during the Vietnam War was made up of draftees. Since college students could get draft deferments, young men from middle and upper class families (those most likely to go to college) were not well represented in the Army. In addition, many of those who enlisted because they expected to be drafted were from this demographic group as well.
The most distinctive thing about the make-up of the US Army during this war, then, was the fact that it was made up of a substantial number of draftees and of poor and working class young men.