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 labor union even if he or she is dissatisfied with the performance of the current labor union.

These organizations may comprise individual workers, professionals, past workers, or the unemployed. The most common, but by no means only, purpose of these organizations is "maintaining or improving the conditions of their employment" ch. I

Over the last three hundred years, many trade unions have developed into a number of forms, influenced by differing political and economic regimes. The immediate objectives and activities of trade unions vary, but may include:

• Provision of benefits to members: Early trade unions, like Friendly Societies, often provided a range of benefits to insure members against unemployment, ill health, old age and funeral expenses. In many developed countries, these functions have been assumed by the state; however, the provision of professional training, legal advice and representation for members is still an important benefit of trade union membership.
• Collective bargaining: Where trade unions are able to operate openly and are recognized by employers, they may negotiate with employers over wages and working conditions.
• Industrial action: Trade unions may organize strikes or resistance to lockouts in furtherance of particular goals.
• Political activity: Trade unions may promote legislation favorable to the interests of their members or workers as a whole. To this end they may pursue campaigns, undertake lobbying, or financially support individual candidates or parties (such as the Labour Party in Britain) for public office.