Please see Computing Services' blog post for more details
All magic is of divine origin. Three pillars of magic (arcane, divine, psionic with incarnum being a subset of psionic).
Lower magic is fairly common. Most communities will have an Adept or two, healing the sick and dispensing wisdom. Larger bodies, especially those with older churches, will often have clerics to tend to them.
Higher magic is about, but not common. Plenty of opportunities to find stuff.
Ritual magic breaks the bounds of what is possible by an individual and some gestures, violating the rules of individual spells or creating entirely new effects.