Magic

Magic is divided into two types: ritual magic, which anyone can learn, and sorcery, which only a few can perform. Sorcery is the original form of magic, raw and untamed.
Even now, millennia after the discovery of magic and how to control and channel it, sorcery is dangerous to use. Few sorcerers successfully learn more than a handful of spells before destroying themselves - and often those around. Sorcerers who survive their early experiments generally find a school or teacher, usually of ritual magic. Because the source of both kinds of magic is the same, a sorcerer who performs or participates in ritual magic enhances the spell's effect.
Ritual magic is derived from sorcery. The first sorcerers, attempting to control their power, discovered that certain movements, words, materials, and symbols made their spells more effective. As non-sorcerers attempted to learn spells, these rituals - unique to each sorcerer - were refined and expanded until any non-sorcerer could use the ritual to cast the spell. It was discovered that, for some spells, more people participating in the ritual meant more effective spellcasting.
The primary constraint for ritual magic is time. Any spell can be cast at any time, but it can take hours or even days, depending on the desired effect and number of trained participants. Some spell-rituals, especially those useful in combat or other instant-response situations, have been simplified and abbreviated such that the spell can be prepared in a few moments (when things are calm and unstressful) and completed within seconds when needed.
Sorcery has a different problem. Because most sorcerers invent their own spell-rituals, some of the simplest effects can take the longest to achieve. Of course, most sorcerer rituals only take a minute or two at the longest. But experimenting is extremely dangerous, and it is not unusual for a child of less than ten years to be discovered as a sorcerer when their earliest spells go awry - often fatally.
A sorcerer's spellcasting abilities are limited by her willpower and stamina. She has the option to prepare her spells ahead, but she can always sacrifice her prepared spells for something else. A non-sorcerer cannot swap prepared spells, but can take the minutes or hours necessary to cast any un-prepared spell when time allows.