Here is a great article from fastcodesign that I think explains it really well.
Here is a an excerpt:
INTERACTION DESIGNER (MOTION DESIGNER)
Remember the subtle bouncing
animation when you pull to refresh in the Mail app on your iPhone?
That's the work of a motion designer. Unlike visual designers who
usually deal with static assets, motion designers create animation
inside an app. They deal with what the interface does after a user
touches it. For example, they decide how a menu should slide in, what
transition effects to use, and how a button should fan out. When done
well, motion becomes an integral part of the interface by providing
visual clues as to how to use the product.
However, as with many of these 'newer' design roles, many people that work in these positions often wear a number of different hats - It can be difficult for a smaller company to justify employing someone solely for Interaction design. So you might find that a visual designer, or UI designer, or someone else is doing the role of an interaction designer and vice versa.