Methane is a colorless, odorless gas. It is the main component of natural gas, used as heating fuel here on Earth, and is largely formed from the decay of plant life. Most of the natural methane in the earth's atmosphere comes from swamps and bogs. It is also produced as animals, from cattle to the smallest microbes, digest their food.
Methane is abundant in the atmospheres of the large outer planets in the solar system -- Jupiter, Uranus and Neptune -- but those worlds do not have the earthlike characteristics that make Mars interesting to many biologists.