Can someone explain to me what a union is. I have a general idea but need further clarification. Also Why do major retailers like Home Depot oppose to a union?

Here's what Unions say about the benefits:

"Union workers are more likely than their nonunion counterparts to receive health care and pension benefits, according to the Bureau of Labor Statistics. 89 percent of union workers (private sector) participated in medical care benefits, compared with only 67 percent of nonunion workers. 84 percent of union workers are covered by pension plans versus 56 percent of nonunion workers. Seventy percent of union workers have defined-benefit retirement coverage, compared with 16 percent of nonunion workers.

Unions increase productivity, according to most recent studies. The voice that union members have on the job—sharing in decision-making about promotions and work and production standards—increases productivity and improves management practices. Better training, lower turnover and longer tenure also make union workers more productive. The agreements reached between unions and employers through the collective bargaining process serve to provide an element of security and certainty, and establish employee privileges and benefits which otherwise may not have been provided by an employer dealing individually with employees. Such agreements become the primary source of employee rights and responsibilities in the workplace and govern such things as wages, hours, benefits and terms and conditions of employment. They are legally enforceable and binding contracts to which the employer, employees, and the union must adhere.

Unions play a pivotal role both in securing legislated labor protections and rights such as safety and health, overtime, and family/medical leave and in enforcing those rights on the job. Because unionized workers are more informed, they are more likely to benefit from social insurance programs such as unemployment insurance and workers compensation. Unions are thus an intermediary institution that provides a necessary complement to legislated benefits and protections."

Employers find these benefits for workers to be detrimental to employers - such as the pretty much unrestrained ability to terminate, change benefits, not provide benefits.