Utility Workers Union of America
AFL - CIO Local 127

Virtually every union contract contains a grievance/arbitration procedure, which is the way the union and the employer tackle disagreements about workplace rights covered by the contract.  Filing a grievance is the equivalent of starting a lawsuit:  you put in writing what you believe another party has done that is contrary to the law, and what action will be necessary to correct the situation.  If after going through a series of procedural steps the dispute is not resolved, then the last step of the grievance process —arbitration—is the equivalent of appearing before a judge to argue the case out and obtain a final resolution, one way or the other.
—Adapted from The Union Member's Complete Guide, by Michael Mauer