Fundamental Spells
Are sold by Diane in Julienne's shop. Most of them may be commonly needed in the beginning or middle of the game.
Enhanced Spells
Are sold by Julienne in Julienne's shop. Most of them are enhanced ones by combining spells and adjusting their magnitude and duration.
Deadly Spells
Are sold by Mystina in Julienne's shop. Those magic spells are so much powerful and require so much magicka. If you will use those, your magicka would need to be enhanced with something like enchanted clothes, which are also sold by Mystina.