Spell Hit Rating: This combat attribute increases your chance to hit with spells. Spell hit is a recent addition to the combat system, having made its debut with Patch 2.0 after TBC launched. Building up a collection of gear with this stat on it will enable you to diminish the amount that an enemy will resist your spells. Cool, huh? Each point in this stat equals a certain percentage based upon your level. There are two talents that increase this stat in addition to gear, Elemental Precision and Arcane Focus.
Spell Damage Rating: The amount of spell damage you have as a mage is how much added power your spells have as they damage their target. There are two types of spell damage bonuses available to you, the general plus to spell damage and those that are specific to a certain school of magic, like fire or frost. There are lots of yummy talents out there that increase this percentage, and in addition consumable items like Blackened Basilisk and Adept's Elixir add an extra punch.
Spell Critical Strike Rating: This statistic affects the percentage that a spell critical strike will happen, those brief moments when your spell deals 150% more damage. This number is at the base, and can be increased by trinkets and some gear pieces. Your spell critical strike rating can be increased by specific equipment, but it takes a lot of it to see much of a difference. You have to have 22.1 spell crit to gain 1% to your chance to crit with a spell.
Without throwing too much numbers at you, let's look at the two general types of PvE fights you encounter as a mage. I say PvE because PvP stats are a glorious world of complication best reserved for another discussion. For simplicity's sake, I am going to discuss the top end numbers, those goals you can shoot for as a level 70 player. If you are not 70 these are still a good benchmark, just realize that you won't be able to achieve these numbers until you reach the level cap.
Basically, it all comes down to a small formula: spell hit > spell damage > spell crit = <3. As you can see, gathering equipment with bonuses to your spell hit percentage is of greatest priority, particularly in boss battles in raids or instances. Being able to land more spells outranks bonuses to damage and crit percentage, but only up to a certain point. As with many bonuses to combat stats, there is a threshold with spell hit, and once you reach said tipping point adding additional points to the stat no longer benefits you. What is that magic number? 202. Secondly in non-boss battles, trash mobs, etc. the amount of spell hit you need is around 165, but this is a number is highly debatable.
It will take quite a while to build up your spell hit rating to 202, but once you do your next priority will be to build up your spell damage so that since you are landing your spells they do more damage as they hit. The last stat to worry about is your spell crit, since spell damage bonuses apply to all spells including those that critically strike. Tacking on some spell crit damage will be nice, but those additional points only deal with a small number of your spells (those that crit), and will increase your aggro issues in combat.