From Envirowiki

Methane is a chemical compound with the molecula formula CH₄. It is a gas at room temperature, and is a potent greenhouse gas (21 times as potent as CO₂. Methane occurs naturally in anerobic decomposition, and is produced in large amounts by industrial agriculture.

Methane can be burnt as a fuel (it's a large component of liquid natural gas), and doing so can actually reduce potential greenhouse effect, if the gas would otherwise escape into the atmosphere. When burned, Methane becomes a mixture of Carbon Dioxide and Water (CH₄ + 2O₂ → CO₂ + 2H₂O)