Exandria is the newest setting for Dungeons and Dragons 5th Edition. Originally a homebrew setting for Critical Role, it became an official published setting by WOTC on 17 March, 2020, under the title of Explorer's Guide to Wildemount. It was first touched upon with the quasi-official splatbook Tal'dorei Campaign Setting, which takes place on a different continent in the same world.
The Tal'dorei Campaign Setting is an all-lore book that provides a history, geographical guide and racial writeup for Tal'dorei. While it had mechanical content (A subclass for Barbarian, Sorcerer, and Monk, as well as several other systems), none of it was official due to it being published by a third party.
The Explorer's Guide to Wildemount is basically the same as the above, though focused on the continent of Wildemount, but also includes a lot of crunch, in the form of three new subclasses (the Echo Knight Fighter and the Chronurgist and Graviturgist Wizard), new monsters, new spells, new magic items, a new magic item subsystem based on the Weapons of Legacy, a new Elf subrace, a new Halfling subrace, two variant Dragonborn races, and reprints of the Sea Elf subrace from Mordenkainen's Tome of Foes, the Orc from Eberron: Rising from the Last War, the Aarakocra and Genasi from the Elemental Evil Player's Guide, the Tortle (the first time it's ever been in an official book!), and the Aasimar, Firbolg, Goblinoids, Goliath, Kenku and Tabaxi from Volo's Guide to Monsters.
Exandria's pantheon is and always has been the Dawn War pantheon from the Nentir Vale, with the addition of Sarenrae from Golarion. In the Explorer's Guide, due to obvious legal issues, Sarenrae has been replaced with an expy called Raei.
Exandria houses all of the Player's Handbook Races, although Sea Elves exist here, as do the unique subraces of Pallid Elf and Lotusden Halfling. Additionally, it's also home to the three iconic Planetouched races (Aasimar, Genasi, Tiefling), Aarakocra, Firbolgs, Goliaths, Kenku, Tabaxi and Tortles, as well as non-evil Orcs, Goblins, Hobgoblins and Bugbears. It's also home to a unique "race" called the Hollow Ones, which are basically revenants of any of the aforementioned races.
One weird thing about Exandria is that, if you read the lore, there's a lot of interspecies shagging going on. Elf/Dwarf couples are a prominent thing in the Diarchy of Uthodurn (although the kids are randomly either dwarves or elves, with some minor aesthetic traits from their other parent), whilst the Half-Elf lore notes that the statblock covers things like elf-minotaur hybrids and elf-bugbear hybrids as well as the traditional elf-human hybrid. Orcs and goblins, however, refuse to interbreed, for fear of what might be unleashed by creating a child bearing the divine curses of both Gruumsh and Bane.
Again, all of the standard Player's Handbook Classes are here, along with three new subclasses based around the magical art of Dunamancy; a field of magic based on manipulating the primal energy of potentiality and actuality, which is known as "Dunamis".