It is the same stuff produced when we pass gas or fart 
Just organic material, like plants, animals, that make this gas when they decompose over time. It is a mixture of gases, but the most important one is methane (CH4) which can be used to make natural gas. This is a great form of energy to power things like cars and trucks. It burns clean producing only carbon dioxide and water, so no pollution.
There is a lot more to it, but I hope you find this helpful.
