Maybe it is a bit boring, but I find useful to divide the adventures I propose to my players in 3 kinds:
Campaign: In the perfect DnD style, a party of adventurers will face many different adventures and quests. The connection between the quests is provided by the setting and the party living them: all the facts happen in the same “plane of existence”.
Saga: In fact very similar to a campaign, a Saga implies a common plot connecting all the adventures, resembling one big uber-adventure. The tipycal example is four Hobbits traveling to save the World.
One-shot: A single adventure, with a start and an end, after which the characters are never meant to be use again.
Why I do this? Because having these definitions I can easily explain to my instructed players what I am proposing to them.
A One-shot is limited in the time: players know they can allocate a few evenings or simply a week end to it. It is also “intense”: losing a session is probably very bad, because you are not enjoying a big part of the story. On the other side, a One-shot is usually a VERY good adventure :) A Narrator can orchestrate very nice ideas in a settings in which he has fewer constrains. Heck, you can even destroy the world :)
A Saga is usually not suited for the more Hask’n’Slash players: too many hints and story elements to remember for many months :) The good part of a saga is that they tend to be EPIC. Your character is going to change the world…if the saga ever reaches the happy end. A saga is risky, a lot of them never develop to the final chapters.
The Campaign is probably the easier to maintain, but maybe a bit less rewarding.
I like making a parallelism:
One-shots are blockbuster movies: Full Metal Jacket, Indiana Jones (yes, good one-shots get sequels :) )
Campaigns are ongoing tv series: ER, JAG or Star Trek
Sagas are tv series with connected episodes: Heroes or 4400, or even the Harry Potter books.
What are you mastering this Friday? A Saga or a Campaign?