What is biodiesel and what advantages and disadvantages does it have compared with normal diesel?

Biodiesel is an organic fuel obtained from different types of seed oil(e.g. sunflower, rape, soya) after a process known as "trans-esterification", that makes biodiesel very similar to mineral-based diesel.
The second question is more complex and would require a fully articulated reply (entire conferences and books deal with the question).
Very basically, the use of biodiesel, on its own or mixed with gasoil, doesn't lead to any marked change, neither from the point of view of emissions nor in terms of consumption.
What favours biodiesel is the fact that, being organic in origin, it is a sort of renewable source , unlike fossil fuels. It therefore produces a lower level of CO2 emissions.
The downside is the cost of production (it should be noted that to make it competitive with diesel the government would have to renounce the duty of about €0.40 per liter that applies to mineral-based diesel).
Availability is also somewhat limited: if all the available land in Europe (i.e. all currently uncultivated land) were given over to the production of the raw materials necessary for biodiesel, it would still account for less than 5% diesel consumption for transport needs.