Old school D&D is a game of resource management. I don't mean merely that resource management is an aspect of the game; it literally is the game. The "game" part of a role-playing game like D&D (distinct from the "role-playing" aspect) is all about deploying resources most effectively to achieve some goal. Virtually every decision during a game session involves acquiring resources, expending resources, or conserving resources. Most of the game's mechanics define resources and the relationships between them. Even the dice-rolling parts of the game are really resource management challenges, as they tell us how rapidly resources such as hit points are used up, and inform such resource-based decisions as whether to use an emergency potion or spell or whether to cut losses and retreat.
Uncertainty is what makes the game exciting, and that uncertainty usually can be expressed in terms of resources: How much is completing this objective going to cost us? Do we have enough? How can we make sure we end up with enough and a margin of error?
There are many different kinds of resources, and their differences and the interactions between them define the mechanical play of the game. Resource use forms a sort of economy within the game. Different resources are more useful or less useful to attaining different goals. Some are very versatile, and some are very specific. Resources can also be exchanged for other resources. For instance, time, a spell, or a potion may be exchanged for hit points.
Resources come in both renewable and non-renewable kinds. "Renewable" here means that the resource is fairly easily replenished by expending time (i.e. time is easily converted into this resource); thus spells are a renewable resource because they may be regained after a period of rest, while a potion is non-renewable, because once it's consumed, it's gone. Hit points are also a renewable resource.
Some resources are used steadily and/or predictably, at the players' discretion. Food, for instance, is consumed at a predictable rate, and may also be expended for other purposes (e.g. distracting a pursuing monster) simply by making the choice to do so. Others are consumed
in more haphazard fashion which is not entirely within the players' control. Those are the instances in which dice are rolled to determine if and when and how much of a resource is used. A party might win a fight without expending any hit points at all if they roll well and their opponents roll poorly, or it might cost them dearly if the rolls (and roles) are reversed.
Below, a partial list and discussion of resources frequently utilized in D&D adventures and campaigns.
The most obvious resource is in the form of coin, but gems, jewelry, and other items of great exchange value count too. Gaining treasure is often the main goal of adventuring. In the XP-for-treasure paradigm of classic D&D, it's the driving force behind character advancement, but it's also an important resource for use during (and between) adventures. Aside from its function of generating XP, treasure is used almost exclusively as a means to gain other resources by purchasing them from other characters or creatures. It may purchase food and water, equipment, labor, information, and access. Often treasure may be expended in lieu of other resources; for example, by bribing monsters the party would otherwise have to fight and expend hit points and spells defeating, or by purchasing passage on a ship to save the resource of time when traveling long distances.
The carousing house rules popular in the OSR add a further element of resource management to the game, by forcing players to decide whether to expend treasure on carousing (or a character-appropriate equivalent, such as research or charity), thus "purchasing" additional power, hit points, etc., or to save it for more direct purchases of equipment, information, and so on. The rules-as-written generally allow them to have their XP and eat it too.
Much common adventuring equipment
is geared toward conserving other resources, particulary hit points and
character lives. Armor directly reduces the amount of hp lost in
combat, while weapons minimize it indirectly by neutralizing foes
faster. Poles and thieves' tools reduce the threat of traps. Even the
humble Large Sack allows greater amounts of resources to be carried.
Other items, such as ropes and grappling hooks, allow access to places the party might not otherwise be able to reach.
adventuring equipment is pretty durable, but there are situations that
can force characters to expend these items. An iron spike pounded into a
rock wall probably can't be recovered in usable shape, nor can the rope
secured to the top of a cliff by which the party descends to the
bottom. Even weapons and armor are sometimes subject to damage or
destruction. It's usually best for the DM to warn players if something
they want to do will cause the loss of a piece of equipment, so that it
becomes an interesting choice rather than a "Gotcha!" moment. "Sure,
you can use your 10' pole to vault over the Floor of Lava. It looks
like you'll make it easily that way, but the pole is probably toast. Do
you still want to do it?"
Food and Water
Rations and water are a
resource to be managed, but one that's often hand-waved because DMs and
players deem that the effort of tracking them does not appreciably add
to the fun of an adventure. Hand-waving food and water removes a great
deal of the pressure of time management, though, especially for
wilderness adventures and other forays lasting longer than a day or
two. Provisions are generally replenished by expending treasure to
purchase them, by spending time to forage for them, or by spending spells to create them.
Character health and vitality are such an important resource that the game provides a special statistic just for tracking them. Things like character class and level and the availability of healing determine whether they're a scarce resource or an abundant one, and whether management of them occurs in the context of individual encounters (such as when hp may be recovered by a "short rest" afterward) or whether it's stretched out over the full length of an adventure. Hit points may be replenished through time, spells, or physical resources (potions or healing herbs.)
A character is typically the very last resource a player wants to spend to achieve party goals - at least, his or her own character! - but heroic sacrifice is a well-established trope of the genre. Henchmen and hirelings might be considered more expendable in order to conserve other resources such as magic and player-character hit points and lives. Of course, the more liberally players expend the resource of henchman and hireling lives, the more difficult it may be to replenish their supply.
Losing party members means losing the utility of all that character's abilities, from the most fantastic to the most mundane. Not only can a dead henchman not inflict a little damage on the orcs while drawing off some of their attacks, he's not going to carry your loot either!
A party member may be recovered through the use of powerful magic, or replaced by rolling up another.
Sometimes a character may be "expended" temporarily, as when a party member is paralyzed, unconscious, confined, or taken hostage, and thus unable to contribute for a time.
The spells of clerics, magic-users, and other spell-casting classes are another resource to be carefully managed. Spells are an extremely versatile resource, which can often be spent in lieu of other resources, such as hit points, equipment, and time. Some spells are expended to restore or augment other resources - healing spells restore lost hit points; create food and water grant extra provisions; summoning spells provide extra "henchmen." Spells are typically replenished by time.
Temporary Magic Items
Potions and scrolls are non-renewable resources which can accomplish just about anything that can be done with magic spells, and sometimes a few things that can't. Because they're non-renewable, some players will hold onto them forever, waiting for the perfect moment when they'll make the difference between glorious victory and crushing defeat. (A moment that's unlikely ever to arrive, and even less likely to be recognized except in hindsight.)
Dungeon delving without light is tantamount to suicide! The number of torches or the amount of lamp oil carried prescribes a limit on the length of time that may be spent prowling about underground. Some DMs go so far as to house-rule away continual light spells and dark vision abilities of elves and dwarves so as not to dilute the fear of running out of light during a dungeon crawl. Light as a resource is an effective way of making time matter, at least while in the dungeon.
This is a wonderful, versatile resource, one that a smart party will be willing to expend other resources to obtain. Good information often pays off more than the cost of obtaining it, as it may allow the party to conserve other resources. Expending a read magic spell to decipher the inscription on a crypt allows players to make a more informed decision on whether to open it. Does it contain treasure that may aid them in their quest, or undead monsters that will grind down their supply of hit points and combat spells? Expending a turn of time to search for secret doors may allow them to bypass needless conflicts on their way to their objective.
Conversely, the characters may have information they do not wish others to know - the resource of secrets. For instance, it may cost the party a lot less in spells, equipment, and hp if the Big Bad Evil Guy doesn't know their plans, and even better for them if he doesn't even know they're coming for him. Lesser resources may be wisely spent in preserving the resource of secrecy to spend when it is to the greatest advantage. Secrets may also be used to purchase other resources.
Often the characters may have information that's of value to someone else, and may use it to gain virtually any other resource which the other party can supply, such as treasure, magic, food, or other information.
Finally, characters may possess or acquire information that someone else does not want them to divulge, which gives them tremendous leverage over that person. Such knowledge is an incredibly powerful resource under the right circumstances, but it's usually a one-shot deal. Once it's used, it can't be recovered.
Reputation and relationships
Players can expend all kinds of resources building up the resource of a character's or a party's reputation and relationships with other denizens of the campaign world. Reputation can be spent judiciously to acquire resources from willing NPCs - the count assigns the party a squad of men-at-arms, the merchant lets them hire a ship and crew at a steep discount, the peasants hide them from the evil sheriff's men, etc. Using it in this way generally entails some maintenance costs if you want to be able to count on help again in the future, of course.
Reputation can also be sacrificed in extreme situations. The king has entrusted them with access to the most secure parts of the castle. He'll never trust them again when they use their access to steal the crown jewels, but maybe that's what it takes to save a PC's family from certain death. The evil warlord's cowardly advisor surrenders because the party has earned a reputation for mercy - a reputation that will be severely damaged after they reluctantly agree to torture him for information. Such uses of reputation are often one time only, and the resource may never fully be restored.
...and then there's the Master Resource...
Besides being a resource unto itself, time affects the use of every other. All too often, time is treated as a costless resource, and therefore freely exchanged for any other that may be in short supply. The infamous "fifteen minute adventuring day" is a direct result of this. Making time too cheap also contributes to the ridiculous conservation of temporary magic items - why expend a potion that's gone forever, if you can use a spell that you get back after a rest?
In fact, time as the Master Resource really deserve a post all to itself...which is coming up next!