[Erydian]

I don't have an exact breakdown, but your realm skills affect your spell points, and that is a factor in what spell levels you can cast (I'm talking about Air, Earth, Fire, Water). How many spell points you have in each realm directly affects what power levels you can cast.

You'll need to raise your realm skills in order to increase your spellpoints, and thereby cast higher powered spells. As Jem suggests, you can do this by casting the spell in question, and in my experience, the higher the power you attempt to cast (the redder the better), the faster your skill improves. I wouldn't bother putting too many level up points into the individual realms - they will raise on their own rather quickly with frequent use.