Mana is the energy of the universe that flows around all things, it surrounds and binds all things into the same plane of existence. This energy is used by sentient beings to perform the art of magic, the art of manipulating forces using mana as a power source.
Maesters are magic users who have learned their art through many years of study. Their study is performed in an Ōrātōrium, a temple of magic and learning. Magic users of this kind tend to be older men who have spent decades learning their craft and incantations. The greatest tool and symbol of a Maester is their Grimoire Tomes, which they keep on their person at all times. The Grimoire act as a conduit for them to channel their powers.
Sorcerers are magic users who use their powers without spell. They use powers like elemental magic. They use can cast quickly but can only directly effect things.
Sages are people who obessessed over something (like perfectionists) for years until they became able to use magic just for that thing specifically. Like a carpenter who became able to manipulate the properties of wood.