Chances are if you're making a fantasy world, or writing a fantasy story, you are probably going to want magic to be a part of it. Afterall, isn't that the whole point of fantasy? A world where the impossible can be possible with the only explanation needed being that hey, it's magic! However, when you're putting magic into your world, you need to be careful with it or large parts of your world will stop making sense, not just in the real world sense, but in the internal consistency sense as well.
So, with this let's take at a few things involving magic that you'll want to consider for your world, and the ramifications it can have.
How Prevalent Is It?
How prevalent is magic in your world? Does everyone know it exists? Or are the stories and legends about it considered just that, the wild fantasies of the over imaginative. If it is prevalent, how likely is someone to meet a magic user? Are they all over the place, or for most people is it just stories, only these stories are known to be real, or at least founded on truth.
If it isn't prevalent, why is it not so? What has been done to keep knowledge of its existence from the world at large? Do people just not know? Or do they think that it is a part of the world that has long since ceased to exist? If it isn't prevalent, who does know that it actually exists? Just the mages? or do the upper strata of society also know of it? How are those claiming magic is real treated? Are they believed? Or are they viewed as charlatans and con men trying to mislead the ignorant and get themselves a small power base of fame and wealth to live comfortably. What happens when they actually do real magic? Will the response be awe? Or will it be fear?
If it is prevalent, once again how much so? How common and wide spread are mages and others who can manipulate magic? How often will people in the larger cities see them? What about out in the country? How many mages are there? Just a few serving as grand advisors? Or are their schools that people from all around go to learn magic or at least more about the world from the magi? Is someone doing magic on the street a crime? Or is it acceptable provided they don't harm anyone? Are the magi even liked for that matter, or are they feared as boogeymen who come to take children away?
Who Can Do It?
Is magic something that anyone can do, provided they study enough and in the right way for it? Or is it something only some people are born with? If you have to be born with it, do you still need training? Or is it like walking and running where you can more or less figure it out on your own? Even if it doesn't require training, are there still schools set up to teach people? If so are those who do not accept the teaching left alone or hunted? Feared as monsters or just the same as any other mage?
If anyone can do it, just how much study is required? Where is the training available? Is this considered a respectable thing to do with your life? Or will people be looking at you like you are willingly becoming an abomination?
This is heavily linked to how prevalent, but the ease of learning magic, and just who can do it will determine your prevalence at least partially. If magic is easy to learn, but anyone can do it, you'll need to figure out another reason for why the prevalence is low if you want it that, perhaps the teachers are just very selective, or there are laws in place to limit who can learn. Just keep it all in mind.
How Powerful Is It?
It seems kind of a silly question, but you need to consider it. Just how powerful is magic in this world? Is it just a few conjurers tricks? Or can a fully experienced mage level an entire army with his wrath? Both are actually more common than you might think, though High Fantasy leans towards the 1 mage vs 1 army = fair contest in a best case scenario. The power level of magic, and the average magic users is important for the world for a number of reasons. The more powerful, the more likely those without the ability to do magic will resent those that can do it, or fear what they are capable of doing. . On the other hand, the lower powered magic is, the less likely it is to be respected by those who feel they have 'true' power.
How powerful magic is will have other impacts in your world, such as covered by the next topic.
How does it affect war?
If magic is in your world, at some point or another it's going to have had an impact on combat, especially if it is powerful. This is important, because if you want your world to be focused more on the epic fights of warriors, than you need a reason for why powerful mages don't just over-run every combat. This can be answered in a number of ways, low prevalency, low power, slow working, any number of ways. Even if it's just the mages hide their power from the rest of the world.
The more active in the world mages are though, the more you need to make considerations for the impact they're going to have on the battlefield. At the very least you want to have a few reasons handy for why they're not just dominating the battlefield every time, especially when you are making mention of all the highly lethal spells that they have in their repetoire.
In all honestly, your magic will probably take a little from each of the things pointed out here, and ultimately all this is is a very specialized re-stating of the rule I gave earlier in world building. When you set a rule or decide something for the world, think it out to it's logical conclusion, do not stop thinking about it when you get to the end of what you want it there fore. Think about it beyond that. If magic users are all pacifists, why do they even have those terrible mass-murdering spells? If they have those spells, and they are all over the place, why do they not just completely control the battlefield everytime they're on it? Does a swordsman even have a chance against a mage in a one-on-one? Think it through all the way, worlds are never as compartmentalized as we wish they'd be.