Fossil Fuels are those formed over geological time from the partly decayed ramins of plants or animals. They include coal, oil and nautrad gas. They can be used only once and are finite. They become exhausted because they can only be replaced over considerable periods of geographical time. For this reasons they are classified as 'non-renewable sources' of energy.
By contrast, renewable cources of energy are those capable of natural regeneration on a human timescale. They provide almost continuous flow of energy