In an arbitration, the arbitrator looks into the legal rights and wrongs of a dispute and makes a decision. Once the arbitrator has arrived at a decision, it is binding on parties whether they agree with it or not. It is very much like the way a court case is decided by a judge, except that the process does not take place in a court room, and it is not open to the public. As in a court case, there is usually a winning and a losing party in an arbitration.
In a mediation, the mediator, essentially, helps parties to settle their disputes by a process of discussion and narrowing differences. The mediator helps the parties to arrive at an agreed solution. He does not decide the dispute. A successful mediation results in an agreement signed by the parties, whereas a contested arbitration results in a decision by the arbitrator himself without the agreement of the parties. In a mediation, there is no such thing as a winning or losing party, because there is no binding decision without both parties agreeing to one.