Viacom was actually split into two different companies in 2005. Both the old and new Viacom are part of a corporation, therefore shareholders are the main owners. The new Viacom is concentrated on movies with picture companies such as; Paramount Motion Picture Group, Dreamworks etc. The old Viacom, now part of CBS, has interests in cable and satellite television ie; MTV and BET. The common shareholder for the old and new Viacom, belongs to a private company called National Amusements.
Viacom has senior management and a board of directors because it's a corporation. According to their website Philippe P. Dauman is the CEO of Viacom. Sumner M. Redstone is the Executive Chairman of the Board and founder. You can find more information here: http://www.viacom.com/aboutviacom/Pages/boardofdirectors.aspx