Explanation:

First-generation biofuels are biofuels made from food crops grown on arable land. With this biofuel production generation, food crops are thus explicitly grown for fuel production, and not anything else.
The sugar, starch, or vegetable oil obtained from the crops is converted into biodiesel or ethanol, using transesterification, or yeast fermentation
Second generation biofuels are fuels manufactured from various types of biomass. Biomass is a wide- ranging term meaning any source of organic carbon that is renewed rapidly as part of the carbon cycle.
Biomass is derived from plant materials, but can also include animal materials.

The feedstock used to generate second-generation bio-fuels should grow on lands which cannot be used to effectively grow food and their growing should not consume lots of water or fertilizer.

The feedstock sources include grasses, jatropha and other seed crops, waste vegetable oil, municipal solid waste and so forth.
The advantage is that, unlike with regular food crops, no arable land is used solely for the production of fuel.
Third-generation biofuels are based on algaculture and algae fuel which can grown on water. Fourth- generation biofuels are made using non-arable land. More research is going in this area