Character Spell Lists
Like your inventory screen, your spell screen is divided into categories. On all screens the first column (the one with the icons) tells you which school a particular spell belongs to and (usually) can identify a spell effect within that school. For example, a Frost-based Destruction spell will have a little snowflake inside the icon. The middle column is the name of the spell and the right column tells you how many magicka points it will require to cast the spell. That number appears to decrease as the skill level in the appropriate school increases. The jury is still out with the final word on that issue, but that's the way it's looking at the moment. If you put your cursor on top of the spell, you'll get a pop-out on the right side that will tell you the magnitude, duration, range and school of the spell. Click on a spell to select it and the next time you hit "cast" (<C> or the middle mouse button by default on PCs - check the manual for XBox 360), you'll cast the spell.
At the bottom of the screen is the Spell Effectiveness. Fromir, here, is having a harder time with his spells because he's currently wearing Light Armor and his skill level with that kind of armor is pretty low (somewhere in the teens, I think). As he becomes more skilled with that armor type, his ability to effectively use spells while wearing it will increase. In the meantime, he's losing about 15% of the spell's magnitude and duration.
Note that when it comes to casting spells, the game checks two things: the amount of magicka you have and your skill level in the appropriate school of magic. Your magicka is checked first. Regardless of your skill level in any given school, if you do not have enough magicka to cast a spell, you'll get a message telling you so. If you pass the magicka test, the game then looks at your skill level. If your skill level is high enough to cast the spell, you'll get a message that says something to the effect of "School: Restoration" (or whatever school is appropriate). If your skill level is not high enough, you'll get a message telling you the minimum skill level required to cast the spell.
On your main screen, the currently selected spell will be greyed out when you do not have enough available magicka to cast it. If the spell icon is clearly visible, though, you can cast the spell by pressing the appropriate key. You can make a selected spell castable (is that even a word?) by either boosting or restoring your available magicka through potions, items, scrolls, or other spells.
Spell effects show in the upper right corner of your screen. In most cases, especially where your attributes and skills are affected, you'll get a message in the top-left of the screen that tells you when something lowers an attribute or skill ("Your Speed is being drained," for example). You can get more information by hitting <F3> and selecting the effects tab.