The Player’s Handbook (PHB) and the Dungeon Master’s Guide (DMG) are the two core rulebooks of DnD. The Player’s Handbook contains everything players need: character creation rules, classes, species, spells, equipment, and the core gameplay mechanics like combat and actions. It’s the foundation for understanding how to play the game.
The Dungeon Master’s Guide is focused on running the game. It includes guidance on worldbuilding, creating adventures, designing encounters, magic items, optional rules, and handling edge cases. It also provides advice on tone, pacing, and adjudicating player choices. Together, these two books form the backbone of DnD.
Supplements expand on the core game. Official supplements from Wizards of the Coast introduce new subclasses, spells, monsters, settings, and optional systems. Third-party supplements do the same, often experimenting more boldly with mechanics or themes.
Some supplements focus on specific worlds, others on tools or ideas. While not required, they can greatly expand what’s possible at your table and keep long-running games fresh.