From the introduction:
As you "play" DxD, one person takes the role of the CM or "Comics Master." In other games, this role would be referred to as GM or "Game Master" and DM or "Dungeon Master. We use "Comics Master" here to help keep the focus on recreating the experience of reading (and living) a comic book!
The CM helps the other players interpret the rules and acts as final arbiter in instances where a rules decision must be made. The CM frames each scene and then calls upon the other players to narrate their participation in the scene.
All other players act on behalf of or narrate the actions of a single superhero. This hero is a player's avatar in the ongoing comic book series. To simplify things we refer to all characters who are not controlled by the CM simply as "Heroes" and to all players who are not specifically the CM as "Players."
The CM narrates the actions of all other characters, peoples, or beings not portrayed by the other players. These characters are collectively known as NPCs or Non-Player Characters, although an NPC specifically intended to battle the Heroes may alternatively be referred to as a Villain.
Play will likely begin with the CM describing a scene of danger or conflict. This might be a scene of potential battle where Villains are depicted robbing a bank. Alternately, scenes can depict a Hero's interaction with loved ones, co-workers, or other Heroes.
Combat scenes are the bread and butter of comic book action and should be fast paced and exciting! Other scenes can help establish motivations for future scenes, provide opportunities for healing or recovery, build relationships with other characters, or further a Hero's ongoing story in any number of ways.