A source of supply, support, esp. one that can be readily drawn upon when needed.
This is a suitable definition, which we will extend for Resource Challenges:
A Resource Challenge is any challenge which involves maintaining the quantity of a resource within predefined tolerances.
Let's talk first about the most important resource in video games: Life. I would describe Life as the primary resource, and the reason that this particular challenge type is so vital to gaming. Whether this resource is plentiful (like the giant lifebar in Mischief Makers) or practically non-existent (Like in any old school Mario game, where getting touched while small kills you), Most games have an inherent life mechanic, expressing your leeway in performing the game's tasks. This resource forces the player to attempt to play as skillfully as possible, because the game will end if they do not manage their most important resource. I would argue that games without life (such as Wario Land 3) demand no skillful execution, and in general are puzzle games, but that's a separate blog post.
There are of course other prominent resources in use today. RPGs have Mana, which the player must maintain if he wishes to use more effective abilities in battle, and Gold, which players must keep stockpiled in order to power up their characters. Strategy games, such as Starcraft quite literally have resources to manage. For a period of time, fighting games were getting crazy with resources, Guilty Gear had the guard bar which filled as you blocked attacks and would give your opponent unscaled damage in their combos. Marvel Versus Capcom 2 had life as well as red life, which was life you could regain by switching out your characters.
All of these resources force certain behaviors on the players. In Guilty Gear, to take an example I'm intimately familiar with, the Guard Bar exists to promote players to go on the defensive.
Now what factors into the difficulty of a resource challenge? There are two main avenues for difficulty, and a third sort of obscure one that I'll cover here. The first metric for difficulty is the number of resources that need to be managed. Most RPGs give you two resources to manage: Hit points and magic points. This is pretty common practice even outside of RPGs, personally I think 3 resources is a nicer number, but that's a separate blog post. More than 3 resources, however, and things start to get fairly difficulty
The second metric is the quantity of the resources. In this case I've always found that scarcer the resource, the more difficult the game is. This is pretty intuitive, the less of a resource you have the more difficult and important your decisions become. In old school platformers, for example, your resource was your life, and you really only had enough life to take one hit. As a result those games were extremely difficult. Compare that with modern shooters where not only do you have plentiful life, but it also regenerates and tops itself off. Life in modern shooters is not a rare resource at all, which may be part of why modern games feel so much easier than older ones.
The third, and more obscure metric only comes in to play when there are multiple resources. The resource's (word I'm going to get from Tommy), or the similarities between each resource, factors in hugely to how hard it is to manage the resources. Funnily, this metric is difficult at either extreme, rather than being a perfect scale. If there are many resources and they function on different scales, and are valuable in different situations, then players have to weigh and balance their "objective" worth in a given situation at a substantial mental calculation cost.
The opposite of the spectrum is the Demon's Souls method. In Demon's Souls, all actions are dependent on the number of souls you've collected. Items you buy from the shop are paid for in Demon's Souls, spells you learn are acquired using Demon's Souls and the only way to level up your character is with Demon's Souls. It forces the same decision making process as above, but in reverse. Which choices are important enough that you should spend your limited resource on, rather than determining which resource is more important in the situation.
The primary application of this challenge is to direct players and to modify their play styles. The applications of it are fascinating to study.
