Biogas is mainly a mixture of methane, hydrogen, carbon dioxide, and hydrogen sulphide. It is generated through the anaerobic (oxygen-free) digestion of organic material.
 
As source material for the technical production of biogas are suitable:

1. digestive residual material containing biosolids like sludge, organic waste, or leftovers
2. commercial fertilizer (manure, dung)
3. systematically cultivated energy crops 

Agriculture offers with points 2 and 3 the biggest potential for the production of biogas. Depending on the origin we can distinguish between sewage gas (from sewage treatment plants), landfill gas (from landfill sites), or biogas (from biogas plants).
 
When the gas originates through natural processes and escapes into the atmosphere without human interference and exploitation – from waters, dung, rice fields, and animal sources like rumen – generally, we speak of natural or marsh gas.
 
Biogas always contains some unwanted components like hydrogen sulphide, which must be extracted before its technical use. Biogas can be exploited as fuel in cogeneration units for the generation of electricity or for heating purposes. In the future, it could increasingly be also used as fuel or, at standardized quality levels, could be integrated directly into the local natural gas net.  
If the produced biogas is used for the generation of electricity, it set free waste heat (engine heat). This kind of energy can be used in-house mainly.