What are tax classes (Steuerklassen) in Germany?

Tax classes (Steuerklassen) determine how much income tax you pay. There are 6 tax classes (Steuerklassen) in Germany^1, 2.

Tax class	Description
I	Workers who are not in category II, III or IV.
II	Single parents who are entitled to single parents' allowance.
III	Married workers whose spouse does not have any income, or whose spouse is classified under tax category V. Widowed workers in the year that follows their spouse's death.
IV	Married couples if both spouses earn a wage and live together in Germany.
V	Married worker whose spouse is in tax class III.
VI	Worker who receives a wage from multiple employers.