(written from a Production point of view)
Viacom is a major media conglomerate that currently owns Paramount Pictures and, as such, owns the rights to the Star Trek film franchise. They also have ownership to Paramount Pictures' home entertainment operations.
Viacom was founded in 2006 following a split between CBS and the former Viacom the previous year. The former Viacom, which previously owned all rights to the Star Trek franchise, was re-established as CBS Corporation. Viacom's television division (Paramount Television), which includes the Star Trek television franchise, fell under the ownership of CBS and was renamed CBS Paramount Television, now known as CBS Television Studios. The new Viacom also retains ownership of Spike, MTV, and Nickelodeon. Viacom, in partnership with News Corporation and MGM, launched Epix, a new pay movie channel, in fall 2007. Star Trek will be run on that channel, as the ten-year exclusive deal between Paramount Pictures and CBS' Showtime expired at the end of 2007. 
The current Viacom and CBS Corporation are both still controlled by National Amusements.