Magic is the mystical energy that infuses all things, and may be harnessed and directed in a variety of ways, from protecting a home, to helping a patch of land grow, to warding off evil, or even forming a fireball. The ability through which this is harnessed is Mojo.
Most 'normal' folks have a Mojo of 4 or less. You use mojo for casting spells, and representing your ability for this. Mojo doesn't necessarily represent how powerful your spells are, but rather how well you have a chance of pulling off a certain spell. You do not watch to botch a fireball. Mystical bloodlines often have a much higher Mojo than other families. In order to use spells of certain types, you must purchase them as background skills.
What kind of background spell skills can you have? Well, that depends on your character concept. When coming up with ideas for spells, please consider your weaknesses and strengths. What happens when you fail a spell cast of a certain type? What sorts of uses does a spell have? Generally speaking, you are responsible for outlining the types of magic your character is experienced with. You buy background skills for the things you are specifically trained in, but can attempt other spells with guidance from a book or other source. Do you craft your own spells, fine! Mojo is the base from which your spell casting is rolled. Background skills are modifiers to this attribute.
Anyone can attempt to cast a spell from a book. The more powerful the spell, the more dangerous the casting attempt. For this reason, only those who are well trained (with a fairly decent background skill in the type of spell being cast) and have a high Mojo, should mess with the very powerful spells. The 'backfires' are not coded, but rather should be RPable.
If you attempt to used a background skill of Summoning to rustle up a full blown demon to assist you in +combat, and you botch, said Demon enters combat as your enemy. Woops, you just made the odds way worse. Most magics of these type require a ritual before hand. When in doubt, consider the Dresden Files. Ritual, ritual, ritual. Don't flub, you won't like the results. The more 'off the cuff' your spell is, the more dire the botch results will be. In a ST run scene, they will choose it for you. In your own, be creative and be true to the theme. Magic gone wrong is bad. That's why no one just plays with it!
Depending on how your background is written, you might be a natural caster. You could purchase evocation, the messy form of magic where you just thrown your will and power into it. This is quite dangerous, but can be harnessed for a variety of purposes. Limitations must be written into your BG. However, the more specific you get, the easier it is on us, when reviewing your app, to approve you. No, you don't have to call it evocation. Dresden uses that term. You can call it something else logical, then explain it thoroughly in your BG so we can figure out what you're talking about.
Note: When applying for powers, please list very specific limits and benefits of your ability. Note whether any focus or ritual is necessary to harness energy, and any physical manifestations of the casting (colors, light, heat, etc).