A mediator is not a judge, and will not hand down any decision. Instead, the role of the mediator is to facilitate a discussion about the issues you are facing, and help you get past the areas where you normally get stuck.
A good mediator asks questions that require both parties to evaluate what is most important to them in a final resolution. He or she will help keep parties focused and productive, while guiding them through the process of crafting a comprehensive, workable solution.