A campaign is created by creating an Epic Arc that has a common thread that progresses through a number of story arcs. Typically the Campaign is geared to last 10 to 15 sessions of play, with the idea of each session overcoming 3 to 5 encounters (in a 4E setting). Typically assume the party will exhaust 60% to 80% of possible Campaign encounters, the remaining encounters can often be salvaged in some form to be used in later story arcs (or future campaigns) with some modifications.
Here is an example Campaign web that has woven 7 minor Quests with a Major Quest for a 14 session, 60 Encounter campaign web.
|S01:||q1I, q1P, q1D, q2I, q3I, <Rest>|
S03: q2F, q2P, q2F, q2C, <Rest>
S04: q2R, Q1I, q3P, Q1D
*S05: q3S, <Rest>, q3F, q3W, q3D
S06: q3C, q3E, q3R, q4I, <Rest>
S07: Q1P, q4P, q4S, q4D, <Rest>
S08: q4W, q4C, q4E, q4R, <Rest>
*S09: q5I, q6I, q5D, q5P, q5S
*S10: q6P, q6D, <Rest>, q5W, q5D
S11: q5C, q5E, q5R, <Rest>, q7I
S12: Q1S, Q1F, q6C, q6E, q6R, <Rest>
S13: q7D, q7S, q7C, q7R, <Rest>
S14: Q1F, Q1W, Q1C, Q1R, <Rest>
Total Encounters 60
Introductions have been bold, story arc climaxes have been highlight in red, and the rewards are shown in gold. The <Rest> designates a long enough break for the party to typically recover and be at full strength afterwards. When possible sessions should be ended with and Rests, to minimize the tracking from one session to the next. This is not always the case, especially if the scenario is designed to be a grueling delve. The "*" sessions above are ones that do not end in a rest, thus extra tacking needs to be done on these sessions to insure that the party is at the correct health and combat preparedness for the next session.
You can see in the above campaign web that the first session is largely a story telling one, where the party gets introduced to three separate hooks that they could possibly act on any order, or maybe the DM pushes them into a predefined order. The "Introductory" encounters are typically non-life threatening by design, and are there to drive the story. Note that the overall campaign arc doesn't even begin until session four, at least formally, though its entirely possible that the party has been dropped some clues that make the formal hook fairly obvious, and possibly even driven by the party's desire to speak to a particular NPC.
The campaign web above was designed as a 4E campaign, where you typically get 3 to 5 encounters before the party will need to rest. For other game systems a session could easily have twice as many encounters, I'll get into the design of these in another post where the campaign definitely becomes more web-like in appearance.
By classifying encounters in this format the DM can easily see the parties progression toward the goal, or really what the intent of the encounter is. Thus, if you have the encounter flagged as "Filler" and the party is taking substantial damage then you may want to adjust the opponents on the fly to making the next hit make them bloodied, or if a "Weakening" encounter is handled way too easily, then before they get a 2nd wind you may want to add another group that were arrive later to the melee.
Plus, it's also nice when prepping which encounters are truly optional, and which are critical. It will be next to impossible to prep an entire campaign out in the form above without completely steering the party into the direction you as the DM want them to go, but if you think the story plays best in a given order having it outlined such as this will give you a direction to push the players into when they seem lost.
Tracking campaigns in this notation can provide the DM with possible clues on when they might "drop a clue" for another story arc, or when they may need to prep a delay encounter, or when they might have enough strength left in them to prep and Extension encounter.
Non-Linear campaigns provide multiple Introductions toward the beginning of the campaign, which include multiple completely optional quests. These are more for groups that contain advanced DMs and advanced players, as the element of free will and having to make choices that matter may not come easily to all players. Some groups like the linear campaign where each encounter lead perfectly into the next, with minimal thought of where do we go now.