Methane is the simplest hydrocarbon, consisting of one carbon atom linked to four hydrogen atoms. It is an odorless, colorless gas produced by cow farts, methanogenic bacteria, and is also found accompanying oil fields.
 In global warming
As well as a fuel, it is a vicious greenhouse gas - around 24 times more effective at positive radiative forcing than carbon dioxide. The only reason that it isn't, under normal circumstances, as much of a problem as carbon dioxide is its relatively low lifetime in the atmosphere, approximately 12 years. Its global warming potential significantly reduces over time because methane oxidizes naturally in the atmosphere to produce carbon dioxide and water. But, as methane is such a powerful greenhouse gas, any rapid increase in its atmospheric concentration would be very noticeable and potentially devastating.
 Methane frozen in ice
Methane that has been stored in ice (as clathrates[wp]) can be released into the atmosphere as ice caps and glaciers melt - accelerating global warming far more dramatically than CO2 could on its own. Indeed, massive events where large quantities of methane have been released from clathrates are suspected to be responsible for some of the warmest periods in Earth's history, and possibly the mass extinction event at the end of the Permian.
The amount of methane stored in ice and water, and the rate that it will be released into the atmosphere is one of the points of contention for scientists trying to model and predict climate change. In the worst case scenario, the anthropogenic component of global warming will cause enough methane to be released to make the damage sudden, devastating and irrevocable.
 It's the cows, stupid
The release of methane by farm animals, specifically cows, has been used as an argument for vegetarianism for several years. However, science has determined that fish oils reduce cow farts. Feed a cow a fish, and it won’t fart for a day. Teach a cow to fish, and it will never emit greenhouse gas again and we're free to eat as many burgers as we like. Sort of.