The primary draft law in the Union during the Civil War required all men between the ages of 20 and 45 to register, with quotas being drawn from every congressional district. Those chosen who did not wish to serve could hire a replacement or pay $300 for an exemption.
Before 1862, neither the Union nor the Confederacy practiced mandatory conscription. The Militia Act of 1792 required all men to own a rifle and join their state militia, but it had never been actively enforced. Legislation passed in 1862 gave states the ability to conscript soldiers, but this was largely used as a psychological goad to encourage free enlistment. The Enrollment Act of 1863 was the first mechanism by which American citizens were actively conscripted into military service.
The substitution and commutation clauses of the Act caused a significant amount of civil unrest. The fact that a rich conscript could simply pay a poorer man to take his place on the front lines led to the common refrain of "rich man's war, poor man's fight." The $300 fee for a commutation was put in place in an attempt to keep substitution wages from skyrocketing out of control. The Act led to draft riots, most notably in New York City.