Reply #16

An agency shop is a form of union security agreement where the employer may hire union or non-union workers, and employees need not join the union in order to remain employed. However, the non-union worker must pay a fee to cover collective bargaining costs. The fee paid by non-union members under the agency shop is known as the "agency fee."

Where the agency shop is illegal, as is common in labor law governing American public sector unions, a "fair share provision" may be agreed to by the union and the employer. The provision requires non-union employees a pay "fair share fee" to cover the costs of the union's collective bargaining activities. The "fair share" is similar to the agency shop, but usually more restrictive as to what may be charged to the non-member.

http://en.wikipedia.org/wiki/Agency_shop