Most people who GM RPGs are aware of the distinction between a railroaded adventure and a sandbox. In the first type of game the story is on rails, and the PCs can't really get off the track. The DM creates a linear set of events or encounters, with each one leading to the next in the series. Most new DMs panic when the PCs try to diverge from the rails, and do everything in their power to steer them back. It almost always feels forced, leaving the PCs at a loss for a sense of freedom (which, after all, is one of the biggest strengths of tabletop RPGs vs video games). In the second type of game, the DM supplies the world which the PCs are free to explore and influence as they desire (much like a child playing in a sandbox, building sandcastles, etc. wherever they desire). There's often not an obvious "hook," and the PCs won't have an employer that meets them in a tavern to tell them exactly what they need to go and do to obtain the treasure. It feels very much like a living, breathing world.
The problem is that railroad adventures are much easier to design because you know exactly what you'll have to plan for. While DMing a sandbox, you need to be prepared for almost anything, and you need to be at least reasonably comfortable improvising. What sometimes gets overlooked, however, is that these two styles are not always dichotomous, and most campaigns exist somewhere between the two on a spectrum. Indeed, this is probably for the best. Not all players have the creativity or motivation to move forward in a sandbox campaign, and even those who can won't necessarily be doing it all the time. Sometimes glimpses of a track are necessary to show players the way, or to get the creative juices flowing. Likewise even the most apathetic of players will sometimes do something unexpected that causes the party to veer off the rails, and it usually works out for the better if the DM just goes with it (in an interesting and engaging manner), before eventually steering everyone back to the railroad.
A good DM will be able to fine tune their campaign depending on their group, or even on how their players are feeling on a given day. In other words, they need to be able to move across the spectrum in either direction without too much effort. Because I personally have more trouble designing sandbox style campaigns, and because my group tends strongly toward railroading when they DM, I'm linking some helpful resources that I found pretty much by chance today.
- The West Marches describes a specific campaign that lies very far toward the sandbox end of the spectrum. The DM is explicitly neutral and has no ulterior narrative motive. He just provides an interesting and detailed world which the players (which are many) are free to explore. This type of campaign honestly sounds like a whole lot of fun. The obvious downside is that there's no narrative structure, but that's by design. It's just a different type of game, really.
- The Slaughterhouse system uses a lot of sleek 4e design elements to organize sandbox campaigns. The world is divided into zones (the author gets bonus points for using Metroid as an example) and zones are populated by factions (the exact meaning of which is left intentionally vague). Each zone gets its own stat block, and are event assigned one of 4 roles (Lair, Outpost, Contested Territory, Uncontested Territory). The stat blocks state who (which faction) occupies or uses a zone, how they use it and how they behave when confronted, their numbers, how the zone can be re-populated, and how its role can change. Since the PCs will be directly affecting these things, each zone actually has multiple stat blocks describing possible outcomes (and since outcomes are very general and relates to the zone's role, this is far less intimidating than it sounds). While I might not use this system exactly as presented, I'm definitely planning on heavily borrowing its elements as a way of providing tangible structure for a sandbox-style adventure. It seems to me that just by actively using it the system encourages sandbox thinking over railroading, and therein may lie its biggest value.