What is the difference between the official language and the working language?

Arabic, Chinese, English, French, Russian and Spanish are the six official languages of the United Nations. English and French are the working languages of the United Nations Secretariat and are used in day-to-day professional exchanges. The treaty bodies, in accordance with General Assembly resolution 68/268, may use a maximum of three of the official languages for their work, with the inclusion, on an exceptional basis, of a fourth when necessary to facilitate communication among the members. Each State party also retains the right to interact with the treaty bodies in any of the six official languages.