Glossary

I got into a bit of a debate with a touchy blogger primarily because he used a different term than I did and thus refused to see the point I was making so I thought I'd specially call out a few terms for clarity. For the first two I'll borrow the definition used by Bearded Devil as it very well written and matches my own.

Prepping - Providing a detailed, thought-through environment for players to explore and inhabit.
Planning - Aiming for a specific narrative arc or set of story beats.

The way I see it planning is the stuff of railroads, story games, and boxed text. Prepping is detailing the sandbox so the GM is ready no matter what madness the players come up with.

Judges Guild Approach - Another term I got from the Bearded Devil post linked above and it goes hand in hand with Prepping. This means providing as much detail as possible. Mapping as much of a city as possible (City State of the Invincible Emperor by Judges Guild mapped the whole city, thus the term) so the characters can go anywhere, do anything. This is the opposite of the Vornheim approach. From my own experience I'd have called this the Harn approach as back in the day the combination of the Harn regional module and Cities of Harn gave me latitude and confidence to let the characters run amok.

Vornheim Approach - Seat of your pants world creation at the table using imagination and tables. This approach is neither better or worse than the Judges Guild Approach. Different GMs will have a different preference of more likely they'll find middle-ground somewhere.

West Marches Campaign - Back in the day Gary Gygax seemed to let different adventure parties exploring Castle Greyhawk instead of having a set group of players and day to play. The different groups would clear out areas and steal treasure before another group explored a location. it was because of this initial style that he obsessed abit about time in the dungeon. Then Gygax went away from this style and moved towards tournament modules. The Knights Errant over at KnightsSemantic have redescovered this style of play and have had great success in the process. Its detailed in the post The West Marches: A Style of D&D Campaign for Large Groups. Even if you don't have a group so large that some are pushing for adventures in the middle of the weak the sandbox setup developed for the campaign sounds exactly like the kind of thing any GM should be aiming for.

Ars Ludi has more detail on the West Marches Campaign in Grand Experiments: West Marches.