Resources can be defined as anything that can satisfy human needs. They can be classified in several ways: natural vs man-made, renewable vs non-renewable, and biotic vs abiotic. Biotic resources come from living things, while potential resources exist but are not currently being used. Renewable resources like sunlight can be replenished, whereas non-renewable resources like minerals form over long periods and cannot be readily replaced once depleted. Human-made resources are natural substances that have been modified by humans to become useful.