Motion graphics vs compositing
Motion graphics consists of Graphic Design and Animation. Main focus is to deal with images, typography, color and illustrations, also it's key to work with design assets, like flow charts, icons, images or backdrops, which created with animation in mind.

However Compositing consists of Animation, Mathematics and Rating Pictures (eyeballing). Main focus is digital image manipulation - it's the art and science to combine (matching) images from multiple sources like film, photographs or 3d renderings into new (blended) images to create a perfect illusion.