Biomass is a form of renewable energy that uses compounds created by living organisms as a source of fuel. Biomass is the first form of non-labor energy harnessed by human society: we've been burning plant materials to warm ourselves and cook our food for as long as 1.9 million years.
Biomass as a renewable energy source is used in a fashion not all that different from the way our early human ancestors used it: wood and other plant material is burned to heat water. The difference between biomass power and cooking or making tea is that we then use the resulting steam to drive turbines that produce electrical power. Biomass isn't widely used as an electrical power source in the US: the largest biomass power plant in the US, which burns agricultural waste from Florida's sugar cane plantations, has a capacity of 140 megawatts.
Of California's 33 biomass power generating stations, the largest is the 50 megawatt Covanta Delano power plant, which burns trimmings from the gigantic industrial orchards in Tulare in Kern counties, as well as wood salvaged from urban landfills. Other facilities in California burn sawmill and other timber industry waste, bagasse and mulch from agriculture, manure and sewage sludge. According to the California Energy Commission, biomass power plants produced 2.1 percent of California's total electricity consumption in 2007.
At first glance, biomass is a carbon neutral energy source because burning it releases only that carbon which the plants being burned took in during life. Accounting for that carbon gets more complicated when biomass power is used as justification for cutting forests. This is increasingly the case in western states like Nevada eyeing their pinyon juniper forests as possible fuel sources. Burning wood definitely releases only that carbon the tree absorbed during its life, but leaving the forest standing keeps that carbon out of the atmosphere altogether. Though plant biologists thought a decade or two ago that some older forests sequestered less carbon than younger ones, increasing knowledge of interconnected forest ecosystems has proven that to be a misconception. Mature forests host communities of plants, fungi and microorganisms that can sequester a large amount of carbon.
Agricultural waste is used as a fuel by many plants, and that's an unquestionably better use of the material than burning it in the field without pollution control -- and then burning fossil fuels to produce the power the waste would have provided. As in the case of forest products biomass, however, there are uses for agricultural waste that do even more to reduce our carbon footprint, such as composting the material and using it to amend soils.
"Waste To Energy" plants, in which combustible municipal garbage is burned to generate power, are often cited as a form of biomass energy. However, waste streams contain a large amount of material such as plastics whose carbon content originates in petroleum and other fossil fuels, and burning them adds to our carbon footprint. While agricultural waste streams are often made up of a few different kinds of materials, Municipal Solid Waste (MSW) streams contain millions of differing substances, and only the most cursory sorting is practical before the waste stream enters the plant. Waste-to-energy plants have long been criticized by environmental justice activists as posing a serious threat of toxic exposure to the communities in which they are sited.
One specific type of biomass fuel, which could sensibly be considered a subcategory of waste to energy, is landfill gas. Organic solid waste that decomposes in the absence of oxygen creates methane, which is highly flammable -- it's the main component of natural gas. Landfills that don't account for methane production in their design can explode, or perhaps even worse, outgas methane into the atmosphere, where it acts as a greenhouse gas 72 times as powerful as carbon dioxide. As even the best-designed landfill will leak its methane eventually, it's clearly a wise move from a climate mitigation perspective to trap that methane and burn it for power, thus not only drastically reducing the methane's danger to the planet's climate but replacing some fossil fuel derived power besides.