The arcana (sometimes known as the weave) is the medium through which all magic is drawn. It is described as a “fabric” that is present on any part of the physical planes, and made up of “strands”. These strands correspond to different ideas in the material world (for example fire, sky, the hearth, chaos, justice, friendship). Every concept has a strand, some strands occur more frequently than others, particularly in certain places, as they are intrinsically linked to the physical world. This does NOT mean the magic derived from these strands is stronger in certain places, due to the interconnectedness of the arcana meaning a tug anywhere is felt by the whole fabric.
The existence of this arcana is considered universal knowledge, and it is accepted that it is the source of all power. However, to most, it is merely a faraway concept that has no effect on their everyday lives.
Casters of magic draw from the arcana. Almost all people can establish a weak connection with enough hard work and devotion, but only to the level of a first level caster. It is therefore quite common to see cantrips used, especially among the higher classes with more access to the time and the education. It is, however, perfectly possible to use magic with no training, especially if you are gifted. Higher level magic, though, is a scarcity, and it’s users are regarded with respect. This does not mean to say that a particular town will welcome a caster; there are many different views on higher magic, usually source dependent.
Divine casters have a strong connection to particular strands (or families of strands), with druids drawing from the more primal concepts and clerics being more connected to social ones. The passive nature of this magic means, so long as you believe you are being true to a strand, you can use it’s magic (once you have established a strong connection to the arcana). Arcane casters are connected to a larger part of the arcana, but not as closely. They are not bound to devotion to the concepts they draw magic from, and as a consequence sometimes find themselves under accusations of exploitation from certain groups of people.