There are many differences between the two. First, snipers only work with a partner, who is usually a spotter. Spotters serve as scouts. Snipers are sent into the field to do one thing, and that is to assassinate high-value targets. Snipers are the best shot too.
A marksman is part of a squad. They also tend to extend the range of the team itself and permit them to engage farther targets. Marksmen aren’t the best shots in a lot of cases either.