Skill Points

Possibly rivalling experience for the most coveted intangible in the game.
Skill points is what allows you to learn spells, learn weapon abilities, and
healing abilities. Skills are on a steep curve, so at the earlier levels, you
can buy something from every skill tree, but quickly, you'll have to cut down to
one or two, and even then, you won't be able to purchase every ability in the
tree you want. This is why people SP farm, so they don't have to choose.
"Eenie, Meenie, Miney, ... Oh why not all of them?"
http://www.gamefaqs.com/computer/doswin/file/930711/43300