What is the difference between humus and parent material?

That humus represents a stage and that organic matter signifies the content is the way in which humus and organic matter are different.

Specifically, the term humus indicates that animal and plant remains as well as soil food web cells, products and tissues are broken down as far as they can. The term organic matter identifies cells, products and tissues related to once-living animal and plant organisms. No assumption will be made as to which stage this once-living content is in terms of breaking down.