Magic always comes with a price. I read the book, you know.
—Henry to David Nolan
Use of Magic
Use of Magic is the ability to harness magic to cast spells, however, using dark magic corrupts the user. Powerful magical practitioners like Cora, Regina and Rumplestiltskin become corrupted by their powers and in turn, their magic turned tainted as well. Not all magical practitioners are corrupted, however, such as fairies, but they are exceptions because they cannot use magic in negative ways, instead of choosing to not do so. ("Once Upon a Time: The Price of Magic")
Magic can be used to override the usual laws of nature, physics, and reality, but it always comes with a price. It is inherently neutral and cannot itself be split into Good or Evil, but can be used for either purpose. Since magic use is powered by emotion, the strength of the spell depends on the feelings of the caster focuses into the output of magic. Negative emotions are generally used to power spells while positive emotions are used to maintain control of spells.("The Miller's Daughter")
However magic can be split into Light and Dark, forms of magic that are powered by positive or negative emotions, thus a manifestation of the wielder's will. However positive emotions do not equal goodness as a mage's attempt to protect one's loved one can result in a person's death- after all, the road to hell is paved with good intentions. ("Kansas")
Laws of Magic
The Laws of Magic define the limits of magical abilities. There are three restrictions even the most powerful magic is said to be unable to break: bring back the dead, force someone to fall in love and change the past. However, these laws can and have been broken and changed by powerful spells. One of them is the combined power of three genies who have themselves overridden fate. This spell even makes the caster immune to the price of magic. Despite breaking the laws of magic, the caster is unable to cause others to truly fall in love, but merely create an illusion of love. Another case of law-breaking magic is the Time Travel Spell that was, in one case, enacted by gathering symbols of wisdom, courage, love and innocence. ("And They Lived...", "Kansas")
Dr. Frankenstein attempted to use science to override the laws of magic and bring back his brother with a heart that had been strengthened against lightning surges by magic. Unfortunately, the end result was a violent, amnesiac former shell of the person Gerhardt had been. In one instance of a proper resurrection, Maleficent is revived after combining the blood of those who greatly wronged her with her ashes. ("In the Name of the Brother", "Unforgiven")
Price of Magic
The Price of Magic is the inevitable expense for using or casting magic. Often stated as a warning that "magic always comes with a price", but sometimes, the price is not foreseeable. Rumplestiltskin's curse has the heaviest penalty for gaining his Dark One powers. While the magic allows him to accumulate masses of power, it corrupts him both physically and emotionally. ("Desperate Souls", "Skin Deep")
Other prices include the Evil Queen's sacrifice of her father to unleash the Dark Curse upon the Enchanted Forest. In doing so, she creates a void in her soul that can never be filled. At another time, Regina's attempt to outsmart and get rid of Emma by putting her under a Sleeping Curse backfires, as Mr. Gold predicted, when Henry is afflicted with the curse instead. Even Mr. Gold himself, who knows all too well there's always a price to pay, brought magic to Storybrooke, which returned his powers, but the cost forces a barrier around town to form, preventing him from leaving to find his son with his old memories intact. When Gerda asked for a powerful potion to erase the memories of her sisters from the citizens of Arendelle, the price may have factored in causing her daughter Elsa to inherit her sister's ice powers. ("The Thing You Love Most", "A Land Without Magic", "We Are Both", "The Snow Queen")
Magic itself often manifests as a puff of smoke, which can vary in colors, depending on the practitioner. Purple seems to be the most common color for the Magic's smoke manifestation. When brought over to Storybrooke, this is how magic looked like. The color varies according to the wielder and/or the form of magic: for instance, most of the time a person sticks to one color (although there are certain instances where, due to unknown reason, their magic color is different). Furthermore, the Dark Curse manifests as a very large black cloudy storm in shades of black and green.