All appropriate compost ingredients are organic. Organic refers to matter that is a part of, or comes from, something that is (or was) alive. In other words, organic refers to plants, animals and their naturally-occurring by-products.
Nature has a cycle for animals and plants. When they die, they decay. Their decaying forms provide food for fungi and bacteria, which provide food for other microorganisms, which provide food for bugs, which provide food for frogs, which provide food for larger and larger creatures throughout the food chain.
Decomposing remains also provide nutrients and structure-building materials to the soil so that plants have a healthy, nutrient-rich environment in which to grow. Composting encourages this transformation from decay of dead forms into food for soil creatures and plants.
Inorganic materials do not decompose in this way and should be kept out of the compost bin.
There are organic materials that should not be composted at home for various reasons, including health, odour, and safety. These materials will decompose, but a home composting system is too small to address problems created as they decay. Effective composting of these materials generally requires the higher temperatures, professional monitoring, and odour control techniques of large-scale composting.
You should always take your household needs and safety into account. Pine needles and cones were listed as an acceptable material for home composting earlier in this article. However, you should not use a load of needles and cones if you are allergic to pine, or children who will be playing on a lawn fertilized with the finished compost are allergic to pine, or if someone has sprayed the pine tree with pesticides.