Workers collectively have greater bargaining power while negotiating terms of employment (salary, benefits, leave, penalties) with their employer. To this end, they form trade unions.
Breakdown of negotiations
When negotiations with employers break down, they go on strike - that is halt work without pay, thereby causing loss to the employers. Employers sometimes try to get around the problem by hiring new workers who will accept poorer conditions and work - which often leads to violence and damage, and hence is an unattractive option.
Administration, financing, membership
Trade unions are usually run by members elected from among the employees. Not every employee is required to be a member of the union or participate in their strikes, but if a union exists and does bargaining on their behalf, even non-members have to pay a bargaining-fee to the union. Trade union elections tend to be very politically charged, and in many cases, they often coerce and punish disagreement and non-membership by illegally means - including arranging undesirable assignments and violence.