Labor is work. Actually labor isn't only work but let's start there. Any work whether paid or unpaid, voluntary or not is labor. We normally consider it to be paid work done for a place of employment. Employer's trade money for time worked.
As industry has increased and our society has become less dependent on farm work the word labor began to be used by unions. Labor Unions are employees that have organized under the supervision of a group who is supposed to ensure a living wage, good working conditions, and freedom from abuse.
When our culture was more agriculture based people labored on the land. They grew crops which were sold and that money was used to purchase goods that were needs that the agriculture workers could not make for themselves.
When the industrial era began people worked in factories. These factories did not always have the best working conditions and were not always safe. Factories which had bad conditions were referred to as "sweat shops". True sweat shops are brutal places where long hours of hard labor are exchanged for minimal compensation. The unions began their rise to power in these conditions.
When people are dissatisfied and treated poorly they will turn to those who promise to help them. It is a natural response in humans and has happened in most industrialized countries. Labor workers usually have good pay and benefits which encourage workers in other places to unionize as well.
I think therefore I am might work in poetry but I eat therefore I survive is what matters when a person is trying to feed and clothe a family. There is no labor too menial that someone will not perform the task if they have a family to support.
Labor is not just the hard work though. Whether the work is in the field or in the courtroom it is all defined the same. There may be varying levels of sweat and mental exercise but it all goes to raise the standard of living for the family.
I believe we've established here that labor is work.