- They start gaining access to spells at 1st level.
- They can reach up to 9th level spells.
- (In 5E) When multiclassing, each level in this class counts towards the character's Caster Level.
Usually, a full-caster's power is offset by physical frailty. Their durability is quite poor when compared to others and their combat ability is considered equally weak sauce, if it even exists at all. Thus, their power is usually better spent augmenting their casting powers or gathering new spells to cast. In practice, divine full casters often have plenty of combat ability out the gate and buff spells can give most full casters ample durability.
Third Edition full casters include Cleric, Druid, Sorcerer, and Wizard in core. Non-Core full casters include Archivist, Favored Soul, Sha'ir, Shugenja, Spirit Shaman, and Wu Jen. Beguiler, Dread Necromancer, Healer, and Warmage all technically meet the definition of full caster, but have very restricted lists that make them stand separately from the others (though the first two are probably better classes than Shugenja, even in terms of raw power). The Psionic classes of Psion and Wilder are considered "full manifesters". Ardent and Erudite might share that title if they were from a book that everyone didn't pretended doesn't exist. While Artificer isn't technically a full caster, it can steal enough of their tricks it's the only class that can even hope to compete with them and often gets honorable inclusion in the term, if only because nobody wants to say "full casters and Artificers" all the time. Pathfinder adds Arcanist, Oracle, Psychic, Shaman, and Witch to the list of full casters.
Full Casters in D&D 5e currently consist of the Bard, Cleric, Druid, Sorcerer, Warlock and (most iconic) Wizard. Artificer is in a unique position of only meeting the first trait, placing it in a new tier commonly referred to as a Two-thirds Caster.