Resistances modify damage taken from a specific damage sources (e.g. puncture, slash, crush, impact). Armor and armor accessories can be found in the treasure system with any type of resistance. This modification is generally positive for the player and results in reduced damage and wounds, but vulnerabilities exist which allow for increased damage and wounds as well. Resistances exist for all critical tables except for UAC-specific ones and noncorporeal, since characters are corporeal.
Resistances to fire (heat), cold, steam, and nature, are available to be applied temporarily on non-metal armors through the ranger spell Resist Nature (620). Fire, cold, and steam resistances will protect from those respective element-based spells and maneuvers. Nature resistance is unique in that it lets the wearer withstand hits from nature-based attacks, which include all offensive Ranger Base spells and Sandstorm (914), as opposed to a specific critical table.
Monks have an inherent ability to MEDITATE to receive resistances. The amount of resistance is 10%, plus a 2% per bonus per a seed 1 summation of the monk's Mental Lore, Transformation skill. Thus, a monk with 28 ranks of ML, Transformation would have a 24% resistance ability, and a monk with 91 ranks would have a 36% resistance.
Loresinging can be used to detect the presence and amount of protection provided, as well as any potential vulnerabilities, but if the source of the item is unknown (i.e. if it came out of a locker after many years), a ranger should ASSESS the item if a natural resistance is detected that can be applied with the Resist Nature spell to determine whether the resistance is temporary and how many hits remain. A loresong will correctly identify that an item is temporarily resistant when it comes from the treasure system, and while resistance applied with Resist Nature will not explicitly be indicated as temporary, the song will usually provide the name of the ranger who imbued the resistance, as well as the wear and tear on the armor fittings. Warriors can also ASSESS armor to determine how many hits are remaining on the armor fittings.
Creature Puncture Resistance
For a time there were many creatures that were immune to puncture attacks. This was replaced with varying resistances to puncture, with bony creatures and arachnids the most resistant to puncture damage, and creatures with thick hides less resistant (more vulnerable).
The Ranger Resistance Scale
The following information was provided by Nuadjha, the Briar Fox and may not be entirely accurate.
This is resistance to natural and elemental damage added by the spell Resist Nature (620).
Armor Resistance Scale
This category covers resistance to: crush, slash, puncture damage; added by a merchant, warrior fittings, or that is otherwise inherent in the armor.
These are just logical guesses based on slim evidence from my few data points. Anyone able to fill in some more points?