Taxation is the inherent power of governments to impose and collect taxes from individuals, properties, and transactions in order to generate revenue for public purposes. There are several principles and theories of taxation, including the benefit principle, ability-to-pay principle, and equal-distribution principle. A tax system can be proportional, regressive, or progressive depending on how tax rates and amounts change relative to income. Taxes are an enforced contribution used by governments to fund programs and services that benefit citizens.