It's dubbed "winter gasoline". Some deny its existence, some know about it, some even know what it really means. Many, however, don't really know much about it besides what they hear. For simplicity, it is called "winter gasoline"- the gasoline that doesn't have to meet stringent summer EPA air pollution laws as those laws are relaxed ahead of colder weather.
Many areas across the United States have different laws for emissions during the hot summer months. Air pollution becomes far more of a problem when the heat and sun cause pollutants in gasoline to become far more of an issue. With cooler weather, it's not such a concern, so those stringent laws are relaxed. What's the difference? Many areas revert back to a more similar standard- even some bigger cities. Chicago, for example, uses a unique blend of summer gasoline that is only used in the metro area- NW Indiana, Chicagoland, and up to Milwaukee. Come winter, that special requirement is gone, and the areas uses the same gasoline as cities outside those boundaries.
The same holds true for many areas. Detroit doesn't have the same gasoline that Chicago burns in the summer, but it is still more stringent that other areas in Michigan.
So what changes between summer gasoline and winter gasoline? The Reid Vapor Pressure number, also called RVP. RVP is a measure of the volatility of gasoline. Areas that don't have as many pollution issues can burn higher RVP gasoline. Areas that have pollution issues burn lower RVP gasoline. Summer gasoline is generally considered to have RVP of under 9.0. Winter gasoline is considered to have a RVP of higher than 9.0, usually 11.5-15.0. The higher the RVP, the more volatility, and more emissions and pollution. That's not to say gasoline with RVP of 15.0 will cause widespread smog, much of it depends on the number of vehicles on the road.
The lower the RVP, the more expensive the gasoline costs to produce. Generally, higher RVP gasoline has more relatively cheap butane (think of a butane lighter), and in gasoline with lower RVP, there's less butane, substituted with more expensive, cleaner burning components.
So as summer winds down and temperatures cool, areas will slowly be reverting to cheaper, higher RVP gasoline. You may notice slight drops in fuel economy, but the biggest story is that there are far fewer mandates and unique blends of winter gasoline, and that means more supply of one type of gasoline, instead of tighter supply of the same fuel with different RVP numbers.
Come Spring 2013, we'll again be talking about summer gasoline being produced, with lower RVP, and that costs more to produce, but for now, its the cheap winter gasoline that we'll transition to beginning widespread on September 15.