Some armour just adds more defence while others add elemental resistance.
You should be able to recognise which armour adds elemental resistance by the set of numbers towards the bottom of the item description. There should be around 5 sets of numbers or something rather each next to a sybol representing the element type.
they belong to different classes. There are 3 sets of armour per level (note: new armour does not come every level): melee, ranger and spiritualist. They differ in elemental defense, defense and requirements.