For this post, I'd like to describe two different idealized styles of play for pen & pager RPGs. The styles are also not tied to any particular rule set, although some rule sets may tend to encourage one style over the other. These styles are idealized in that most actual games being run will contain some elements from each.
In a Rules-Centric game, the focus of play is on the game rules, both the general rules as well as any module-specific rules. Much like a board game, the rules lay down the parameters for the characters' interaction with the game world and players state their actions using the vocabulary of the rules, as opposed to the vocabulary of the game world.
In a World-Centric game, the focus is on the game world, while the rules are just a tool for supporting play in that world. The options for character action are as limitless as the game world allows and players state their actions in the language of the game world.
The Computer Game Test
A simple test to check where a games falls out on this spectrum is to answer the question "How easily can it be translated to a computer game?" If it is Rules-Centric, then the translation is simply a matter of writing a program to enforce the rules. If it is World-Centric then the translation will be impossible, since the limitless possibilities of the gaming world cannot practically be expressed to the computer.
ImmersionPerhaps the most important difference between these two play styles is how they effect immersion. Immersion is a subjective experience, however in my experience, World-Centric play tends to be much more immersive than Rules-Centric play. This for the simple reason that focusing on the game-world, rather than on the rules pulls you deeper into that world.
In my opinion, this is one of the major arguments for the continued existence of pen & paper RPG's in the computer-age. For all the graphics, algorithms, voice acting, etc., pen and paper RPG's still have the potential to provide a more immersive experience.
There is a trend in game design to produce endless supplements and encyclopedic rule-books covering every possible situation real or imagined. Since rules define the limits of the game world, Rules-Centric games have a much greater need for these additional rules, so that a greater range of situations can be covered. World-Centric games on the other hand, are not limited by their rule-set and the GM(possibly with the help of the players) can just make up a suitable mechanic when needed, so the need for supplements is much less acute.
Mapping to GNS
GNS is a well known theory of game styles based on what players want to get out of the game.
If we had to map these styles to the GNS model, I would say that the Rules-Centric style roughly corresponds to Gamism in that much like a traditional game, the rules define the limits of the game. In this sense, the Rules-Centric style sees an RPG as a highly complex board game.
The World-Centric style is also largely Gamist, but it is a very particular kind of Gamism: Immersionist Gamism. A World-Centric game is unlike traditional games in that the goal is not winning via the rules mechanics, rather on creating an immersive experience for the players. As such, the rules take on a secondary importance.
"It's very 'gamist', but also in its own way very immersive"