How would you describe the concept of sandbox play?
How about to someone who is used to adventure modules or adventure paths?
As with anything, usage varies, but usually when people say "sandbox" today they mean a campaign that does not have a specific prescribed storyline, but one where the GM sets up a world (or at least a small section of one) and the PCs are free to wander where they will and find adventure where they will. It's about freedom of player choice.
Pure sandbox play is purely simulation driven. A super hardcore sandboxer places a dungeon (or whatever) in the game world and that's where it is, for the PCs to come across or not (and for NPCs to come across before them or not). If a thief is sneaking into a mansion, in a sandbox game he is able to avoid guards and traps, and not have predetermined plot points presented to him regardless of his actions.
Sandbox is not an antonym for adventure module. Some of the early modules, most notably Keep on the Borderlands, were extremely sandboxy, as were many of the early dungeons (Castle Greyhawk, etc.). Here's a place, there's fell monsters and treasure there, go do what you want. More recently, Paizo did a sandbox-style adventure path called Kingmaker for Pathfinder. Sandbox is a different approach from story-driven - a "story of what happened" may emerge from a sandbox session but a preconception of story, or what "the GM wants to happen," is never applied to the game. Adventure paths, being a series of adventures, can try to be sandboxy but generally try to provide enough story to get PCs from one chapter to another, but event timelines and things like that can serve that purpose without being railroads (though people often complain and call things like that railroading, just because they feel pressured to do something).
Railroading, the antonym of sandbox, is simply extreme constraint of choice. Some perceived constraint of choice is always there in any simulated world in that there are always choices that are impossible to physically perform or clearly undesirable, but where you cross the line to railroad is when these things are obviously being imposed by the GM/metagame (usually in the name of "The Story" or "The Plot").
You can be apparently providing a sandbox but using the game world to provide so many restrictions that you are effectively railroaded into a single course of action. A dungeon full of one-way doors that inhibits all teleportation and divination, for example.
Most games are somewhere on the continuum between pure sandbox and railroad, or even move between the two based on need and GM inclination. Many campaigns switch back and forth between railroad and sandbox. Railroading to move the story on when the players lose momentum and sandbox otherwise it a frequent GM tactic that lets the players be free when they want to be but gives them structure when they're feeling lost.
Sandbox gaming can be desirable because it produces a sense of game world reality that enables the player to focus less on the metagame and immerse in their character and the game world. It can be problematic because players can feel like they are spinning their wheels and wasting limited leisure time without more guidance, and because sometimes a preplanned story cam have more "big, interesting" things happen in it plot-wise than a sandbox.
I tend towards sandboxy play, but in my most recent campaign I had players get frustrated and ask for more direct guidance from me on "what they should do" - I am normally reluctant to do that but did so to make them happier. Often players want the illusion of sandbox and unlimited choice, but with the GM pulling strings behind the scenes to keep them headed towards interesting things.
The term originated in computer games and it's meant to describe a game where its playing field is wide open for the player to do what they want. Around 2005 with the release of Necromancer Game's Wilderlands of High Fantasy Boxed Set, its authors—I am one of them—used it to describe to people what made the Wilderlands different from other settings. It was designed to make it easy for the referee to adjudicate his players roaming freely across the map.
Later still, the term got attached to a specific playstyle as mentioned by mxyzplk. However this is beyond what myself and other Wilderland authors intended. The problem is that people take the hard-core simulation of wandering the map too literally. This often results in frustration as many PC groups feel rudderless and the game feels without direction. In fact, if you read through various forums posts, such as on ENWorld, you see these campaigns fail more than succeed.
The trick to overcome this is "World in Motion." You work with the characters to give them a background they like in the setting. This provides a framework in which the players can make their initial choices. This background can incorporate what some consider railroad elements, like being members of a noble household, a guild, a temple, etc. But the key difference is that the players are free to leave or ignore those elements, as long as they are willing to suffer the consequences.
Along with this you develop a timeline revolving around NPCs and events. This timeline is created with the idea that this is what happens if the players didn't exist in the campaign. This timeline becomes your plan. It gets altered as a result of the consequences of the players' actions. At some point the campaign will become self-driving as the consequences of the consequences start propelling the players forward.
Again the Sandbox was meant to describe a type of setting, not a playstyle. But you can't control how these things go on the internet, so hence the confusion.
People often get confused due to the prior use of Sandbox in campaign. The term "Sandbox" was used for RPGs prior to the development of the Wilderlands Boxed but for other aspect of gaming than a type of campaign and setting. Some examples include:
Dragon #25, Tim Kask
He still clings to the shibboleth that wargamers are classic cases of arrested development, never having gotten out of the sandbox and toy soldiers syndrome of childhood.
Dragon #247, Page 123
Grubb has a phrase for working with existing games, settings, and characters: playing in other people's sandboxes.
Later in the issue
Having gone freelance three years ago, Grubb has explored new sandboxes. I worked on Mag Force 7's Wing Commander and Star Trek (original series) trading card games, ...
In this issue "sandbox" was used interchangeably with how most roleplaying gamers use campaign.
The key to a sandbox is to give the players the choice in what challenges they tackle. "Adventure module" isn't the opposite of "sandbox", either. It's completely possible to use purchased adventure modules in sandbox play.
In fact, purchased modules make things a lot easier, since they provide a lot of focused material to populate a sandbox quickly and you don't always know which way the players are going to jump. To use some classic examples, if the players decide to hire up as mercenaries to defend the free lands against the scourge of the Slave Lords, then you can run them through the four-part Slave Lord adventures. If, instead, they decide they want to tackle (literally) bigger challenges, they can head north to fight Against the Giants. Either could lead to Descent into the Depths of the Earth, or, if the players don't want to follow up on those clues, to something else entirely.
That freedom to go where they want and choose the challenges they tackle is what makes a game into a sandbox.
Great question - as I have a long history of running plot driven games, and I've decided to freshen things up by stressing a simulationist/sandbox style in a Pathfinder campaign I started last year. Most of my players are used to plot driven games, so it took some getting used to.
I explained it as follows:
You are part of the world, and while naturally there are constraints based on your character background as you've designed him, and while there will be adventure hooks littering the landscape, you are not obliged to bite any one of them - EXCEPT how you want to direct your character. Keep in mind that means some areas may be below your "level" or some may be above your level, so good perception skills are even more useful.
From a simulationist perspective, all NPCs will be presented according to their background and their intelligence - even modestly intelligent creatures with some emotional control can be willing to talk it out. They are not pre-programmed based on an overriding plot line.
A few important things with simulationist play:
Here is an article I enjoyed reading by Gabe from Penny Arcade.
It covers his experience as a GM trying out sandbox play after being used to more linear adventures, including how he approached developing the world and encounters the players might find.
Also: pretty pictures!
An excellent article that I read once (source long forgotten) holds up the video game Red Dead Redemption as an archetypal example of a sandbox setting. In the game you can go off and explore to your heart's content; hunt, ride, interact with random NPCs. However, to move the plot forward, you have to talk to one or more important NPCs who are available to hand out quests.
In my sandbox campaign, there are about half a dozen NPCs who are quest givers, some more reliable than others. I basically prepare three adventures for a given session and link them to one or more of the NPCs in a way that is logical, meaning that the setting and encounters will remain the same but the goal may vary based on who gave the quest. Over time this three-adventure approach means that I have a reserve of adventures ready should the party talk to the right NPC at the right time.
To keep things moving, there are some events that simply are going to happen in the world, whether the party participates in them (to help or hinder) or ignores them. These events will have consequences for the setting, the NPCs, and the party themselves. Basically the world doesn't wait for them to act.
One important skill that you must have if you're running a sandbox is the ability to improvise-- an interaction, a skirmish, or an entirely new plot line. Try as you might to predict what the party will do, they will always find a way to throw you for a loop.