"The federal government collects various fees from businesses and households. Choosing to raise public funds via user fees, as opposed to other means such as taxes, has important administrative and economic consequences. Many fees stem from 'business-like activities,' in which the government provides a service or benefit in return for payment. For example, many national parks charge entry fees, which then help fund maintenance projects. Such fees and charges that result from voluntary choices, such as entering a national park, are distinguished from taxes-- which stem from the government's sovereign power to compel payments. The Government Accountability Office (GAO) defines a user fee as a 'fee assessed to users for goods or services provided by the federal government. User fees generally apply to federal programs or activities that provide special benefits to identifiable recipients above and beyond what is normally available to the public.'"
CRS Report for Congress, R45463
Congressional Research Service: https://crsreports.congress.gov