Old school pencil-and-paper RPGs (Dungeons & Dragons and its ilk) have the unit of a game as an "adventure", maybe known as a "module". An adventure takes a few hours to play through, and generally advances your character a level or two. The theory being, you'll get some gaming buddies together for a couple of evenings of play, and play through the adventure. If, after that, you got together with a different group of buddies, you can still use your beloved character, and just play a different adventure.
Classic D&D module "Tomb of Horrors" (http://home.flash.net/~brenfrow/dd1/s1.htm):
Typically adventures are completely self-contained; they have a location or two (maybe a town and a dungeon), some sort of overall objective, and once the players obtain that objective, the adventure is over, and time to move on to the next one. Often multiple adventures share a setting (Greyhawk is the old-school D&D setting, but there are many others), but nothing more; no continued story or shared antagonists or anything (but see further down for exceptions).
This type of structure is rare in the CRPG world, for the simple reason that adventures are generally too short and unsatisfying for a full game. I played through "Witchs Wake", a Neverwinter Nights module that more or less fits the Adventure definition, and definitely felt unsatisfied and wanting more when it ended. (In that particular case, it wasn't helped that the story was clearly meant to continue but was never completed).
Witch's Wake (http://nwn.bioware.com/gallery/index.html?galleryID=7&screensize=2&screenimage=8):
Bag of Adventures
One thing a game designer could do is to simply release a "game" with a bunch of adventures, rated for different levels. The player could then pick and choose which adventures to play, based on his current character. There are actually some advantages to this, such as player-controlled difficulty scaling; if the player wants a less challenging adventure, he just picks a lower level, or maybe shorter adventure for his next task. However, you really lose the sense of continuity by just presenting the game as a series of unrelated adventures, and I've never seen a game take this approach.
One place that this might work well is as a multiplayer-only, online game. You get together with some buddies and play through an hour long adventure in an evening. The next night, you get together with some different buddies, and play a different adventure. I suppose this is something like playing an MMO that uses instanced dungeons, although MMOs have a whole extra set of interaction that goes far beyond this idea. Playing a Diablo game online is more what I'm thinking of, except of course Diablo has an overall structure to the game that is not broken down into nicely quantized pieces of evening-long gaming.
Diablo 2 (http://www.gamespot.com/pc/rpg/diablo2/images/0/2/):
So, an adventure is too short to be satisfying; why don't we just link up a series of adventures to make the overall game? And in fact, this has been done, both in the pencil and paper world, and the CRPG world.
A few years ago, Dungeon magazine (the D&D adventures magazine) came up with the idea to link together a series of episodic adventures, published once a month, into an overall storyline; they called this an Adventure Path. The first one was called "The Shackled City", but it's been so popular that this practice has continued, both in the online Dungeon magazine and with Paizo's Pathfinder Adventure Paths. (Paizo was the old publisher of Dungeon magazine, which ended for reason too complicated to be interesting here, but readily available online).
Shackled City Hardcover (http://paizo.com/image/product/catalog/PZO/PZO1000_500.jpeg):
This format has clear advantages. The episodic nature means that the story is tightly controlled, so the game designer can tell a dramatic, well-formed story without worrying too much about the player wandering off track. The designer has a good idea of what level the player is at each step, so he can tailor the difficulty appropriately. And the players get the satisfaction of recurring characters, long-term objectives, and all those other things that are possible in a longer, more-developed campaign.
I find this format basically equivalent to some of the linear RPGs that have come out for computers; something like Icewind Dale. Now, Icewind Dale didn't explicitly sub-divide the game into a series of adventures, but it effectively did so; as a player you progress from one location or objective to the next, with no choice in the matter.
Icewind Dale (http://www.gamespot.com/pc/rpg/icewinddale/images/0/2/?tag=screenshot):
There are drawbacks to the Adventure Path format. The player may feel railroaded, with little choice about what course he progresses on through the game. It is difficult to present a good exploration interface in an Adventure Path, because the game typically doesn't want the player to have choices about which location to go to; if there is an exploration phase, it's usually just an illusion where the player "explores" but there is only one location to find.
The game designer can take a somewhat more flexible approach; I saw this referred to as a "Super Adventure" in the latest D&D Dungeon Master's Guide. To quote "A super adventure is a type of short campaign- really one long adventure- that focuses on a single, limited setting." Some of the characteristics are:
- The adventure takes place in a single setting (likely an extensive one).
- The adventure allows some non-linear exploration.
- The adventure involves different quests, objectives, and possibly even expeditions.
A classic old Super Adventure is the old adventure "Temple of Elemental Evil":
Temple of Elemental Evil (http://home.flash.net/~brenfrow/dd1/t1-4.htm):
Interestingly, some of the old D&D "Super Adventures" like "Queen of the Spiders", I would probably now characterize as an Adventure Path in the parlance of this article; they are much more linear than I see a Super Adventure being.
So, with the Super Adventure, we're giving the player a much freer reign in which actions and choices he makes; at its best he has complete control over what options and directions he takes. BUT, he can only do so within a limited setting with limited characters. This allows the game designer the ability to create a detailed setting to work in; he won't end up creating whole cities or dungeons that the player will never see.
A computer based example of this is "Pool of Radiance"; the old one, not the remake. This is a big game, with exploration and non-linear choices, but is all set within one city. (Actually I haven't played the remake so maybe that is a Super Adventure too).
Pool of Radiance (http://www.coverbrowser.com/covers/c64-games/24):
Open World Exploration
The very other end of the spectrum from the Adventure Path is for the game designer to give up all control of the player's actions, and just present a world for the player to do whatever he wants. Putting the greatest emphasis on the "role-playing" aspect of the game. Bethesda is the CRPG master of this, with games like Morrowind:
I think the PnP equivalent is not a published adventure at all, but a Dungeon Master who presents the setting, and then lets the players do whatever they want, making up the details as he goes.
Anyways, these games, done well, are incredibly immersive and addictive. They are also a game designer's nightmare; huge amounts of content to create, horrible balancing issues, difficulty in presenting any sort of coherent story to the player. As a player, you have to temper your expectations, sometimes; if you are trying to do something that the designers didn't plan for, you might end up disappointed that you can't achieve your goal, or that doing so ends up being boring or pointless.
To my mind, the very best of CRPGs have taken something of a hybrid approach. The combine a fairly large world or area to explore, along with a more structured plotline to follow. Tricky for the game designer to get right, but very rewarding to play. I would point to the Baldur's Gate Series or some of the Ultima games as examples of the hybrid approach done well.
Baldur's Gate (http://www.bioware.com/gallery/index.html?galleryID=28&screensize=3&screenimage=4):
So, there's a survey of the various structures a designer can take when designing the plotline of a CRPG. In writing this up, I found it really interesting how many great games I could think of in each category. As long as the game designer is careful in fitting his game progression and plotting to the overall structure, he can succeed with any of these approaches.
The RPG Anvil column deals with the design and development of CRPGs, and is published whenever I can convince my lazy ass to type one up.