From WWE.com:
Thirty Superstars from Raw and SmackDown LIVE will fight for sports-entertainment immortality when the annual Royal Rumble Match takes over WWE Network on Sunday, Jan. 29. The winner of the massive melee, now in its 30th year, will go on to challenge for a World Championship in the main event of WrestleMania this April.
A tradition dating back to 1988, the Royal Rumble Match starts with two participants, and new Superstars entering the ring every 90 seconds. This continues until all 30 Superstars have entered the bout. Elimination occurs when a Superstar is thrown over the top rope and both of his feet hit the floor. The last Superstar in the ring will be declared the victor.
So far, the following the Superstars have been announced for the gigantic match: