Organic material is found in nature, naturally. Examples are leaves, animals, grass cuttings, wood, and manure. A good way to think of it would be if it's living or came from a living thing then it's an organic material. For example, rocks would not be an organic material because they are neither living nor from a living thing. Similar inorganic materials would be air and water.
Humus is a form of soil that is made from dead plants, leaves, and animals. It's formed by these organic materials decomposing and being broken down by the life forms living in soil (like worms and bacteria). It comprises the top few inches of soil, typically, and has a lot of nutritious material that plants use to grow.
I hope that helps you! (: