Resources were more used in PW past times (mainly through a universal "power jelly" resource) but as of late they have been much more of a focus to Medieval style armies.
One of the more useful resources, iron is usually the material used for weapons and armour...Of course there are all sorts of different metals but most of them are just for aesthetics rather then gameplay.
Usually for decoration or sometimes weaponry, gold isn't quite as common as iron and even in medieval based maps is not always present.
Usually wood is conjured from the air, but still exists...Very few maps (even medieval based ones) contain trees due to their hard-to-edit nature. Wood is also needed for many things, so is often not collected even when there are trees (due to impracticalities)
Similar to wood, stone is usually conjured from the air, but is pretty much always present in a map...Bases tend to grow so large that continually collecting rock would prove tedious. Due to this, stone is usually an aesthetic.
Acid is a much less common resource but is similar to lava - very rarely used for useful purposes and is more of an obstacle.
Lava is present in most partially underground maps, and comes in two different ways - crusted or non crusted. Crusted lava can be walked upon by light infantry, whereas non crusted has no hardened rock. For it's uses see Acid.
Crystals are a somewhat common resource, and often have special effects - from incredibly strong materials to light focusing energy beams...crystals allow for a diverse resource.
Usually diamond but can be anything else, precious materials are a rare-ish resource and often are only used for decoration or very specific gameplay (such as the need for a certain material to cast a spell/summon a creature
Oil is as common as lava, if not more. It is usually used as a source of power, or as a flammable material. Beyond that however Oil is useless.
Similar to Iron and Gold, coal is a semi-common resource that can be used as a source of fuel, most notably by early modern armies. Else, it is flammable and crumbly.
The most ancient of the resources is barely used at all in modern PW. Being the source of all power - pure energy in jelly form, power jelly is the ultimate resource. (And possibly the most lazy.)
Can be used for many things like: flooding, drinking, putting out fires Etc.