Humus is organic matter that has evolved. Humus through the process of mineralization supplies plants with the nitrogen from the soil. A soil without humus is an inert or mineral soil. That’s why organic matter should not be below 2% in an average soil (with 15 to 20% clay content).
There are other types of organic matter: organic fertilizers, plant debris, fresh manure … that will then evolve through different ways into humus form (see OMSI, Organic Matter Stability Index) … And finally, the term “organic matter” covers all of these materials from natural origin that make up living things.