The Secretary-General of the United Nations (UNSG or just SG) is the head of the United Nations Secretariat, one of the six principal organs of the United Nations. The Secretary-General serves as the chief administrative officer of the United Nations. The role of the United Nations Secretariat, and of the Secretary-General in particular, is laid out by Chapter XV (Articles 97 to 101) of the United Nations Charter. 

As of 2017, the Secretary-General is António Guterres, appointed by the General Assembly on 13 October 2016. 

The Secretary-General was envisioned by U.S. President Franklin D. Roosevelt as a "world moderator", but the vague definition provided by the UN Charter left much room for interpretation by those who would later occupy the position. According to the UN website, their roles are further defined as "diplomat and advocate, civil servant, and CEO". Nevertheless, this more abstract description has not prevented the office holders from speaking out and playing important roles on global issues to various degrees. Article 97 under of the United Nations Charter states that the Secretary-General shall be the "chief administrative officer" of the Organization, but does not dictate their specific obligations. 

Responsibilities of the Secretary-General are further outlined in Articles 98 through 100. Article 98 states that they shall act as the chief administrative officer "in all meetings of the General Assembly, of the Security Council, of the Economic and Social Council and the Trusteeship Council, and shall perform other functions as are entrusted to him by these organs". They are also responsible for making an annual report to the General Assembly. According to Article 99, they may notify the Security Council on matters which "in their opinion may threaten the maintenance of international peace and security". Other than these few guidelines, little else is dictated by the Charter. Interpretation of the Charter has varied between Secretaries-General, with some being much more active than others.
When?
13 October 2016