A trade union or labor union is an organization of workers. The trade union, through its leadership, bargains with the employer on behalf of union members (rank and file members) and negotiates labour contracts with employers. This may include the negotiation of wages, work rules, complaint procedures, rules governing hiring, firing and promotion of workers, benefits, workplace safety and policies. The agreements negotiated by the union leaders are binding on the rank and file members and the employer and in some cases on other non-member workers.Most unions claim a right of exclusivity. The union has the authority to determine who may be a member of the union and who may not. Most unions assert a right to mandate that only its members, and no others, may be permitted to work at certain jobs. Furthermore, the union contract is exclusive with regard to the employer, an employer is generally not permitted to seek out the services of another labor union or hire another competing labo
Source:
http://en.wikipedia.org/wiki/Trade unions