Well... I offered, you guys said you wanted it, and now I'm here to deliver. Be warned though, it's a lot to read, and a lot to take in at once.
Sheep's guide to the magic system of the Fate series.
A few things to clarify before we start:
Magic is different to magecraft. Another name for magecraft is thaumaturgy, and I will be calling it that for the rest of this description so as to not confuse anyone.
Magi (singular magus, possessive magi's? Magus'? I dunno, which sounds better?) are practitioners of thaumaturgy.
Magicians are practitioners of magic.
Prana, Mana, and Od:
Magic Circuits:
Magic Crests:
Thaumaturgy:
Bounded Fields:
Magic:
Summoning and maintaining a Servant:
Command Seals:
A note on the servant Caster:
Finally, a couple of things to consider. Magic is generally a secret kept from everyone. If someone witnesses a battle between servants, that person needs to die. Also, servants have two forms, their physical form and their spirit form. The spirit form is invisible and incorporeal, but they can still communicate and have full access to their senses. It is easier on the prana though, and convenient when not in combat situations.
Alright. That was a lot of work, but I hope it clears some stuff (or a lot of stuff) up, and teaches you some important information (and lots more besides). If anyone still has any questions, feel free to ask.
Edit: As if it was possible I forgot something. I was going to include the ritual for summoning a servant, but I guess I'll put it down here instead.