Resisting Spells Skill
Magic Resistance, or Resisting Spells, is a must for every player, especially if you find yourself in a PvP situation.
The most obvious effect of having magic resistance is on your natural resists. With 100 magic resistance every one of your resists is automatically 40%, even while you are naked. For a new player that is a huge benefit, since you will not be finding good armour for a while. You can just wear armour that only has physical resist (a spined leather set, for example). Raising the skill to 120 gives you 44% in every resist.
Keep in mind that the natural resist benefit does not stack with armour. If you have 40% natural resist, wearing a piece of armour with 10% physical resist does nothing at all. You will not feel any benefit until the armour resists are higher than your natural resists.
A less obvious but equally important use of magic resistance is the ability to resist spells. The main spell affected is paralyze, a Magery spell. If your magic resistance is low, a single paralyze from a master mage will keep you paralyzed for a very long time. And during that long paralysis, the spells that follow can be lethal. Low magic resistance also makes you an easy target for the poison spell. Having high magic resistance sometimes lets you shrug off the poison spell entirely. Magic resistance also gives strong resistance against paralysis fields, bard despair songs, and the Mysticism spells bombard, sleep, spell plague, purge magic and nether cyclone.
Skill Training
The fastest way to train this skill is to repeatedly cast the magery spells clumsy or weaken on yourself. For faster gains, your guild mates can cast spells on you.