According to Encyclopaedia Britannica, dry gas is primarily methane, whereas wet gas also contains a considerable amount of heavier hydrocarbon compounds like ethane, propane and butane. The heavier compounds are condensable; they are frequently separated from the methane and sold separately. In the United States, for a gas to be classified as wet, it must contain more than 0.1 gallon of condensables per 1,000 cubic feet of gas.
Penn State's Marcellus Center for Outreach and Research explains what makes gas wet or dry. Natural gas composition varies based on thermal maturity, the measure of how much temperature and pressure has been experienced over time. Dry gas is more thermally mature than wet gas. The center asserts that the heavier compounds in wet gas must be separated from dry gas because the natural gas sent to consumers needs to have a consistent energy output.
Once the heavier compounds have been condensed and separated, they may be sold together as natural gas liquids, or they may be separated further and sold on their own. Ethane, for example, can be converted into ethylene, which is used to make plastics. Encyclopaedia Britannica reports that "propane and other lighter compounds may be marketed as liquefied petroleum gas (LPG), and heavier hydrocarbons may be made into gasoline (petrol)."