What is the difference between a union shop and an agency shop?

A union shop is a type of workplace where employees are required to join the union or pay union dues within a certain period after being hired. In contrast, an agency shop allows employees to choose whether or not to join the union, but they must still pay a fee to cover the costs of collective bargaining and representation. Essentially, the union shop mandates membership, while the agency shop allows for non-membership but requires a financial contribution.