Overworld uses a modified version of the Amber Diceless RPG System by Erick Wujcik. It will be diceless, and if you are familiar with the ADRPG, it will be very familiar to you. However, there are some differences.
1. Overworld uses the concept of a dual world. Therefore the player will in concept have two characters. They will share some stats, but others will not apply when crossing the boundaries between Overworld and the Real World.
2. While in Overworld, there are properties that will set the PCs apart from others they encounter there. But the characters will not be as overwhelmingly powerful as they would be iin an Amber game.
3. While in the Real World, the characters are competent- overly so- in their field. However, the characters are no longer as head-and-shoulders above the competition as they are in Overworld. For the survivability of the PC, one must keep this fact in mind.
For more information on the game, please see the wiki, specifically, the rules section and the character creation section. Also, please peruse the background of the game for more information on this dual-world the characters inhabit. The complete text of the rules are downloadable here.