A carbon tax is levied on the carbon content of fossil fuels. The term can also refer to taxing other types of greenhouse gas emissions, such as methane| Tax Foundation
A user fee is a charge imposed by the government for the primary purpose of covering the cost of providing a service, directly raising funds from the people who benefit from the particular public good or service being provided. A user fee is not a tax, though some taxes may be labeled as user fees or closely resemble them.| Tax Foundation