Short answers, pulled from the story.
Tasha's Cauldron of Everything was published on the 17th of November 2020. It is a sourcebook for the 5th edition of Dungeons and Dragons, supplementing the Player's Handbook and Dungeon Master's Guide from 2014.
Tasha's Cauldron of Everything adds 26 new subclasses, two or three for each of the twelve existing character classes. It also integrates the Artificer as a full class, bringing three subclasses from Eberron: Rising from the Last War and one new subclass.
Tasha's Cauldron of Everything introduces optional rules that let players decouple their character's racial identity from their ability score bonuses. Players can assign those bonuses to any ability scores they choose, rather than accepting the fixed racial bonuses in the 2014 Player's Handbook. Jeremy Crawford stated in November 2020 that the original racial bonuses existed not for game balance but to reinforce archetypes going back to D&D's origins in the 1970s.
Tasha's Cauldron of Everything sold 105,022 copies in November 2020, its first month on sale. It ranked number two in Publishers Weekly's Hardcover Nonfiction chart for the week ending the 21st of November 2020. In USA Today's list for the week of the 26th of November 2020, it reached number five out of 150 titles.
Wylie Beckert painted the alternate cover of Tasha's Cauldron of Everything, sold exclusively through local game stores. She hid a tribute to the spell Tasha's Hideous Laughter within the art as a scroll crawling with laughing imps and skulls. The standard cover was painted by Magali Villeneuve.
Much of the content in Tasha's Cauldron of Everything was developed through the Unearthed Arcana public playtest program, in which Wizards of the Coast releases draft rules for community testing. Jeremy Crawford stated that of the subclasses tested through Unearthed Arcana, "almost every single one made it into the game."