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.
Originating in Europe, trade unions became popular in many countries during the Industrial Revolution, when the lack of skill necessary to perform most jobs shifted employment bargaining power almost completely to the employers' side, causing many workers to be mistreated and underpaid. Trade union organizations may be composed of individual workers, professionals, past workers, students, apprentices and/or the unemployed.
Over the last three hundred years, trade unions have developed into a number of forms. Aside from collective bargaining, activities 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.
- Industrial action: Trade unions may enforce strikes or resistance to lockouts in furtherance of particular goals.
- Political activity: Trade unions may promote legislation favourable 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. In some countries (e.g., the Nordic countries and the Philippines), trade unions may be invited to participate in government hearings about educational or other labour market reforms.
The origins of unions' existence can be traced from the 18th century, where the rapid expansion of industrial society drew women, children, rural workers, and immigrants to the work force in numbers and in new roles. This pool of unskilled and semi-skilled labour spontaneously organised in fits and starts throughout its beginnings, and would later be an important arena for the development of trade unions. Trade unions as such were endorsed by the Catholic Church towards the end of the 19th Century. Pope Leo XIII in his "Magna Carta"—Rerum Novarum—spoke against the atrocities workers faced and demanded that workers should be granted certain rights and safety regulations. Industries like textile mills and railways companies had started in India in the latter half of the 19th Century.
Origins and early historyTrade unions have sometimes been seen as successors to the guilds of medieval Europe, though the relationship between the two is disputed. Medieval guilds existed to protect and enhance their members' livelihoods through controlling the instructional capital of artisanship and the progression of members from apprentice to craftsman, journeyman, and eventually to master and grandmaster of their craft. A trade union might include workers from only one trade or craft, or might combine several or all the workers in one company or industry. These things varied from region to region, based on the specific industrialisation path taken in the place in question. 
Trade unions and/or collective bargaining were outlawed from no later than the middle of the 14th century when the Ordinance of Labourers was enacted in the Kingdom of England. Union organizing would eventually be outlawed everywhere and remain so until the middle of the 19th century.
Since the publication of the History of Trade Unionism (1894) by Sidney and Beatrice Webb, the predominant historical view is that a trade union "is a continuous association of wage earners for the purpose of maintaining or improving the conditions of their employment." A modern definition by the Australian Bureau of Statistics states that a trade union is "an organization consisting predominantly of employees, the principal activities of which include the negotiation of rates of pay and conditions of employment for its members."]
Yet historian R.A. Leeson, in United we Stand (1971), said:
Recent historical research by Bob James in Craft, Trade or Mystery (2001) puts forward the view that trade unions are part of a broader movement of benefit societies, which includes medieval guilds, Freemasons, Oddfellows, friendly societies, and other fraternal organizations.Two conflicting views of the trade-union movement strove for ascendancy in the nineteenth century: one the defensive-restrictive guild-craft tradition passed down through journeymen's clubs and friendly societies, ... the other the aggressive-expansionist drive to unite all 'labouring men and women' for a 'different order of things'.
The 18th century economist Adam Smith noted the imbalance in the rights of workers in regards to owners (or "masters"). In The Wealth of Nations, Book I, chapter 8, Smith wrote:
As Smith noted, unions were illegal for many years in most countries, although Smith argued that it should remain illegal to fix wages or prices by employees or employers. There were severe penalties for attempting to organize unions, up to and including execution. Despite this, unions were formed and began to acquire political power, eventually resulting in a body of labour law that not only legalized organizing efforts, but codified the relationship between employers and those employees organized into unions. Even after the legitimization of trade unions there was opposition, as the case of the Tolpuddle Martyrs shows.We rarely hear, it has been said, of the combination of masters, though frequently of those of workmen. But whoever imagines, upon this account, that masters rarely combine, is as ignorant of the world as of the subject. Masters are always and everywhere in a sort of tacit, but constant and uniform combination, not to raise the wages of labor above their actual rate[.] When workers combine, masters ... never cease to call aloud for the assistance of the civil magistrate, and the rigorous execution of those laws which have been enacted with so much severity against the combination of servants, labourers, and journeymen.
The right to join a trade union is mentioned in article 23, subsection 4 of the Universal Declaration of Human Rights (UDHR), which also states in article 20, subsection 2 that "No one may be compelled to belong to an association". Prohibiting a person from joining or forming a union, as well as forcing a person to do the same (e.g. "closed shops" or "union shops", see below), whether by a government or by a business, is generally considered a human rights abuse. Similar allegations can be levelled if an employer discriminates based on trade union membership. Attempts by an employer, often with the help of outside agencies, to prevent union membership amongst their staff is known as union busting.
 The International Prevalence of Trade UnionsThe prevalence of unions in various countries can be assessed using the measure “union density”. The definition of union density is “the proportion of paid workers who are union members”. Thus, union density provides a rough picture of union membership only; it does not account for the circumstance that in some countries, also many persons under education, many unemployed persons, many retired persons and/or many persons who had to leave work due to occupational injuries may also be union members. (In some countries, such groups of persons may be strongly motivated to maintain union members if, e.g., educational, unemployment, retirement and/or even disability benefits are in part or totally union-administered.)
Trade union density figures are provided below for countries in every continent on the globe: