Not a problem, very briefly…
Generally skills can mix and match to your own liking, though you get only one point on level-up some skills require a certain level before another increase in the same skill…kind of makes you spread the skills a little. Wasn't too happy about this at first - you can hold skills allotment until required. Some skills are 'passive' and cannot be linked to the quick slots. Some offensive and such skills require the target to be highlighted and in view.
There a small video window which shows the skill in action prior to selection.
EDIT: One nice touch is that some skills can be adjusted down or removed (re-allocated) i believe but havn't tried this yet.
http://www.gamerevolution.com/screen/view.php?game=11715&screen=8
Skill tree…reading left to right in column mode (some further skills lower in the colums cannot be seen in this screen shot.)
Priest.
1 Summon undead. 2 Fear. 3 Charm. 4 Curse. 5 Life tap. 6 Summon demon.
7 Summon ghost. 8 Hide in shadows. 9 Summon mastery. 10 Blind. 11 Master herbalist. 12 way of the wise wizard.
Mage
1 Magic misile. 2 Mana efficiency. 3 Confusion. 4 mana leech. 5 Firewall.
6 Fireball. 7 Polymorph. 8 Magic blast. 9 Destruction. 10 Healing. 11 Way of the battle mage.
Warrior
1 Whirlwind. 2 Jump Attack. 3 Defensive posture. 4 Life leech. 5 Regenerate. 6 Death blow.
7. Rush attack. 8 Fatality. 9 Battle rage. 10 Bleed. 11 Thousand strikes. 12 Reflect.
Ranger
1 Poison arrows. 2 Ranger strength. 3 Ranger stealth. 4 Splitting arrows. 5 Explosive arrows. 6 Way of the ranger.
7. Ranger surprise. 8 Stun arrows. 9 Potion effeciency. 10 Evade.
Dragon Slayer
1 Mindread. 2 Wisdom. 3 Lockpick. 4 Encumbrance.
5 Unarmed expertise. 6 Single handed weapon expertise. 7 Dual-wielding expertise. 8 Sword and shield expertise. 9 Two handed weapon expertise.