A good tax system should meet five basic conditions: fairness, adequacy, simplicity, transparency, and administrative ease.
Fairness, or equity, means that everybody should pay a fair share of taxes. There are two important concepts of equity: horizontal equity and vertical equity.
Horizontal equity means that taxpayers in similar financial condition should pay similar amounts in taxes.
Vertical equity is just as important, however. Vertical equity means that taxpayers who are better off should pay at least the same proportion of income in taxes as those who are less well off. Vertical equity involves classifying taxes as regressive, proportional, or progressive.
While no system of taxes is perfect, it is important to seek horizontal equity because taxpayers must believe they are treated equally. It is just as important to seek vertical equity so government does not become a burden to low-income residents.
Adequacy means that taxes must provide enough revenue to meet the basic needs of society. A tax system meets the test of adequacy if it provides enough revenue to meet the demand for for public services, if revenue growth each year is enough to fund the growth in cost of services, and if there is enough economic activity of the type being taxed so rates can be kept relatively low.
Transparency means that taxpayers and leaders can easily find information about the tax system and how tax money is used. With a transparent tax system, we know who is being taxed, how much they are paying, and what is being done with the money. We also can find out who (in broad terms) pays the tax and who benefits from tax exemptions, deductions, and credits.
Administrative ease means that the tax system is not too complicated or costly for either taxpayers or tax collectors. Rules are well known and fairly simple, forms are not too complicated, it is easy to comply voluntarily, the state can tell if taxes are paid on time and correctly, and the state can conduct audits in a fair and efficient manner. The cost of collecting a tax should be very small in relation to the amount collected.