The raw materials that provide the requisite amount of energy to perform various functions of life are called sources of energy.

For example:
(i) We need heat energy to cook food.
The various sources are: Fuels, wood, coal, kerosene, cooking gas etc.
(ii) We need energy for our transport.
The various sources are: Petrol, diesel.
(iii) We need energy to light bulbs, tubes, to run radio, television and other power appliances.
The source is electricity.
(iv) Production in factories and agriculture also need energy.
The source is diesel and electricity.

Characteristics of a good sources of energy are:

(i) it should give an adequate amount of net energy or work per unit volume or mass,
(ii) it must be convenient to use so as to give energy at a steady rate,
(iii) it must be easy to store and transport,
(iv) it must be economical, and
v) it must be easily accessible. 

Energy sources can be classified as: 

(i) Non-renewable resources: The sources of energy which are produced under special circumstances over millions of years due to slow changes. The process involved is not continuous.
E.g., Fossil fuels like coal, petroleum and natural gas are non-renewable energy sources. With the present rate of their consumption, it is estimated that known deposits of petroleum in our country will be exhausted in another 200 years and that coal may last for another 250 years.

(ii) Renewable sources: The sources of energy which can be harnessed into energy so long as the earth derives its heat and light from the sun are renewable sources. 
E.g., flowing water, wind, tides, ocean waves and biogas are examples of renewable sources. Wood is also a renewable source of energy.

It is true that energy can neither be created no be destroyed but, is converted to one form of energy to another. The fuss because of the fact that, energy in the usable form is dissipated to the surroundings in the less usable form.

For instance, when a candle burns it gives heat, light and other substances but the heat, light and these other substances cannot be put together to give wax to be used again.