MU Rebirth is a remake of MU Online with a heavy PvP emphasis. The level cap will remain at 400, although Webzen says that it is significantly speeding up the leveling process so that players will hit that number in two hours. Yes, you read that right. This plays into the "rebirth" feature, which allows players to go back to level 1 while retaining gained stats.
The closed beta test of MU Rebirth will run from July 3rd to the 10th. Webzen will be hosting a global PvP tournament during the test, although details for this contest are forthcoming.
[Source: Webzen press release]