Methane is a gas that consists of one atom of carbon and four atoms of hydrogen. Biomethane is methane that has been produced from organic matter by bioenergy. Producing methane is called methanation. In biomethanation methane is being produced with methane microbes.
Biogas is being produced by microbes dissolving organic matter. Biogas consists of 60 percent of methane and 40 percent of carbon dioxide.
Synthesis gas or simply syngas is a general term for mixture of gases that consist of carbon monoxide and hydrogen. Wood gas is syngas.