master of ceremonies

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A person who acts as host at a formal event, making the welcoming speech and introducing other speakers.
• noun A performer who conducts a program of varied entertainment by introducing other performers to the audience.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun someone who acts as a host at a formal event
• noun who conducts a programme of varied entertainment by introducing performers to the audience
• noun A rapper

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a person who acts as host at formal occasions (makes an introductory speech and introduces other speakers)