What's the purpose of this legislation? To reduce the gases linked to global warming and to force sources for power to shift from fossil fuels, which release heat-trapping gases when burned, toward cleaner sources of energy such as wind, solar and geothermal.

How does the bill accomplish this? By placing the first national limits on emissions of heat-trapping gases from major sources such as power plants, refineries and factories. These limits put a price on the pollution, raising the cost for companies to continue using fuels and electricity sources that contribute to global warming. This gives them an incentive to seek cleaner alternatives.

Is this the cap-and-trade idea that has been in the news? Yes. The first step in a cap-and-trade program sets a limit on the amount of gases that can be released into the atmosphere. That is the cap. Companies with operations that are covered by the cap will then receive permits for their share of the pollution, an annual pollution allowance.

This bill initially would give the bulk of the permits away for free to help ease costs, but they still would have value because there would be a limited supply. Companies that do not get a big enough allowance to cover their pollution would either have to find ways to reduce it, which can be expensive, or buy additional permits from companies that have reduced pollution enough to have allowances left over. That is the trade.

Other than costs being passed along to consumers, will this affect most Americans' day-to-day lives? It will fundamentally change how we use, produce and consume energy. This bill will put smaller, more efficient cars on the road, swap smokestacks for windmills and solar panels, and transform the appliances you buy for your home.

How quickly will we notice these changes? Some will occur more quickly than others. For instance, measures to boost energy efficiency in buildings and appliances are the low-hanging fruit that do not require major infrastructure changes or new technologies. Other changes are decades off and probably will come when the cap gets more stringent and permits get more expensive.

Why is it so important to tackle global warming anyway? Left untended, scientists say, global warming will cause sea levels to rise, increase storms and worsen air pollution. For these reasons, the Environmental Protection Agency recently concluded that six greenhouse gases pose dangers to human health and welfare. And politically, without U.S. action, developing countries such as China probably would not agree to mandatory pollution limits.