Spoilers for Disciple for Magic Job and Class quests.
Magic is aetheric manipulation but there are a couple different ways to go about it:
Thurmaturges manipulate thier own aether to cast spells.
Conjurers use ("borrow") the aether of nature to cast spells. You can technically fuel conjury only from your own aether, but it will eventually kill you.
Arcanists use pre set patterns (arcane geometries) in their grimore's to manipulate their aether into summons and spells.
Black Mages use aether of the planet to fuel their spells
White Mages were created as a counter to Black Mages and also uses the planet's aether to fuel spells
White and Black Magic seem to use the plant's aether to a much greater extent than any other form of magic we know as the over use of these two magics are believed to have caused the Sixth Umbral Era.
Scholars use magic to bind "other worldly creatures" -- fairies to their control.
Summoners Use aether captured from defeated primals to create egis (summons).
Aether sickness has been mentioned but I have not come across anything saying what it is or how to treat it.