Classes

Full Casters
Earn First Spell Slots at 1st Level, and can eventually cast 9th Level Spells.
 * Bard
 * Cleric
 * Druid
 * Sorcerer
 * Wizard

Pact Casters
Full Casters, in the sense that they earn their First Spell Slots at Level 1, and can eventually cast 9th Level Spells. But, until they can cast a Level 6 spell, all their Spell Slots are of the same level. (At Levels 1-2, all spell slots are First Level. But, at Levels 9-20, all spell slots are Fifth Level)
 * Warlock

Half Casters
Earn First Spell Slots at 2nd Level AND/OR they don't earn spell slots higher than Fifth Level
 * Artificer
 * Paladin
 * Ranger

One Third Casters
Earn First Spell Slots at 3rd Level
 * Arcane Trickster (Rogue)
 * Eldritch Knight (Fighter)