First of all:
Skills use Stamina and Dexterity
Spells use Magicka (Mana) and Intelligence
Spells or skills can use 1 or more different damage types: fire, ice, lightning, water, light, dark, force, poison
Each of these types can inflict different status problems, for example fire can burn an enemy, causing him to loose health each round. They can also slow down an enemy or decrease his defense/attack.
Besides of that every skill can get a scripted special effect for example draining the enemies mana or adding bonus damage to the next skill.
Skills and spells can be aquired by completing quests, increasing stats or randomly after watching enemies using them. But you can not learn all enemy skills. (simply because a bite skill for the player would be stupid)
I am also thinking about giving each skill a certain level. The more often you use the skill, the higher its level gets and maybe you can learn some more skills by leveling up others.