There is a very informative page at the Union of Concerned Scientists website that talks about the various types of biomass that can be used for fuel. Trees and grasses apparently are the best crops for energy. Trees can be harvested without cutting down the entire tree so that they will grow back. Prairie grasses such as Switchgrass, big bluestem, and other native varieties that used to cover the plains are a good source of energy and are perennials that do not need to be replanted and do not require the huge amounts of fertilizer and pesticide that regular crops require--also increasing the net energy gained from these crops.
A new approach to using biomass is to burn it mixed with coal in power plants. Utilities in New York and Wisconsin are experimenting with this approach as a way to reduce carbon dioxide emissions.
A number of noncombustion methods exist for converting biomass to energy. They convert raw biomass into a variety of gaseous, liquid, or solid fuels before using it. The carbohydrates in biomass, which are compounds of oxygen, carbon, and hydrogen, can be broken down into a variety of chemicals, some of which are useful fuels.
Biomass energy systems are likely to be smaller than their fossil fuel counterparts, because it is hard to gather and process more than this quantity of fuel in one place. This has the advantage that local, rural communities -- and perhaps even individual farms -- will be able to design energy systems that are self-sufficient, sustainable, and perfectly adapted to their own needs.
Very green indeed.