Dungeons & Dragons is host to a number of popular, intricate settings for players to explore during their adventures, each in essence a fully fleshed-out fantasy world. For campaigns, DMs can use any setting built for the current handbook, create their own, or adapt older ones to fit current D&D rules. The most obvious locations for the 5e handbook are the default Forgotten Realms, as well as Eberron, Ravenloft and Exandria. However, there are many Dungeons & Dragons settings newer players may have never heard of before.
The first published Dungeons & Dragons campaign, Palace of the Vampire Queen, was released in 1976 and is set in the Misty Isles. Not much is known about this location, though, as it wasn't used beyond the Palace of the Vampire Queen, The Dwarven Glory and The Misty Isles campaigns. Some better-known classic settings, like Dragonlance, have received or will receive 5e remakes. Others, such as Greyhawk, are heavily referenced throughout the 5e handbook and are starting to make appearances in the multiverse. The 2019 module Ghosts of Saltmarsh is the first official Greyhawk adventure published for 5e.
Still, many of the original Dungeons & Dragons settings remain a mystery to those who exclusively play 5e. These make up an important part of D&D's history and could provide insight or inspiration for 5e players and DMs.
D&D Settings New Players Likely Haven't Heard Of
Mystara
Mystara was once an important location, forming the Standard Dungeons & Dragons setting while Greyhawk was relegated to Advanced D&D. It was first used in the 1981 module Isle of Dread, though at that time it was only called The Known World. Much of Mystara's appeal came from its encouragement of player exploration; as the game's base setting, it allowed DMs and players a lot of freedom to customize their experience. Mystara was slowly phased out when the second edition came out and hasn't officially been used since the release of 3e.
Planescape
Planescape is important to D&D's history because, as its name suggests, it brought the concept of a multi-planed universe to the game, ultimately paving the way for 5e's current multiverse. It was first published for Advanced Dungeons & Dragons and later served as the setting for critically acclaimed CRPG Planescape: Torment.
Spelljammer
Spelljammer, a 2e setting, introduced science fiction to Dungeons & Dragons, then an exclusively fantasy experience. The setting takes players to outer space, even going so far as to incorporate real-world sciences as part of its design. Spelljammer is characterized by its technology, most notably its eponymous, magically-powered ships that allow travel between planets. Spelljammer ships can be found in 4e and 5e campaigns, albeit rarely, giving 5e players a glimpse into this setting's world.
Dark Sun
In conjunction with Spelljammer's science fiction is Dark Sun's apocalyptic fantasy, which is closer to a magical Mad Max than the Tolkien-esque world Dungeons & Dragons players knew up to that point. Many fans of the setting are hopeful Dungeons & Dragons will resurrect Dark Sun, partially fueled by a 2017 tweet from 5e co-creator Mike Mearl, as well as comments made about the setting's Mystic class at the 2018 Gary Con.