Spellpower, unlike attack power, is more of a pool of damage you draw from for your different spells. Every spell has a different coefficient (how much it draws from the pool) that is related to its cast time and function. i.e Fireball uses more spellpower damage than Frostbolt because it has a longer base cast time (and Frost has a snare), so it hits much harder. Instant cast spells generally use much less spellpower than ones with cast times.

While attack power is pretty static in its contribution and the melee special attacks generally use a percentage of weapon damage (which is modified by AP), spellpower = weapon damage (and not AP) in that the different spells use percentages of it for their damage.

http://www.wowwiki.com/Spellpower

Nice Torumin, I never thought of SP in that way. Nice to find a unique perspective on old mechanics now and then.

Most of the time, you can head to Wowhead and find the spellpower/whatever coefficient for any spells you're interested in. It can give you a good idea why certain spells hit harder for you, and what you should look at in times of desperation (do I heal myself before I finish the monster off, or will Exorcism do enough damage to take it out?)