Explanation:

First Generation Biofuels - They are produced directly from food crops. The biofuel is ultimately derived from the starch, sugar, animal fats, and vegetable oil that these crops provide. It is important to note that the structure of the biofuel itself does not change between generations, but rather the source from which the fuel is derived changes. Corn, wheat, and sugar cane are
the most commonly used first generation biofuel feed stock.
Second Generation Biofuels - They are also known as advanced biofuels. What separates them from first generation biofuels is the fact that feedstock used in producing second generation biofuels are generally not food crops. The only time the food crops can act as second generation biofuels is if they have already fulfilled their food purpose. Jatropha-based biofuels are included here.
Third Generation Biofuels - The term third generation biofuel refers to biofuel derived from algae. The algae are cultured to act as a low-cost, high-energy and entirely renewable feedstock. It is predicted that algae will have the potential to produce more energy per acre than conventional crops.Algae can also be grown using land and water unsuitable for food production,
therefore reducing the strain on already depleted water sources. A further benefit of algae based biofuels is that the fuel can be manufactured into a wide range of fuels such as diesel, petrol and jet fuel.
Fourth Generation Biofuels - They include photobiological solar fuels and electrofuels. They do not require the destruction of biomass.