Military records can be very interesting to explore.  I do have the history on my Great Grandfather and I know that he only had to fill out the draft card and never went into the military.  He was a husband, father, and farmer in Coon Rapids, MN

In the United States, there were three rounds of draft registration in 1917 and 1918. The first two rounds included men from 21 to 31 years of age; the third round expanded the registration to men from 18 to 45 years of age. Draft registration was mandatory for men living in the United States, regardless of citizenship status.

A total of 24.3 million American men registered for the draft (about a quarter of the population), of which about 2.8 million were drafted. Another 2 million men volunteered or otherwise enlisted (those who had already volunteered did not have to fill out draft registration cards). In all, about 4.3 million men were actually mobilized, and about 350,000 were casualties, of which about 126,000 were killed. http://wiki.answers.com/Q/Was_there_a_military_draft_in_World_War_1

 The best way to find out if you have a veteran in your family is of course to talk to your family.  That isn't always available to most of us.  Search engines are great at finding the missing pieces.