Short answers, pulled from the story.
Gary Gygax wrote the original Unearthed Arcana, with design contributions from Jeff Grubb and editing by Kim Mohan. It was published by TSR in 1985 for use with Advanced Dungeons and Dragons first edition rules.
The book was notorious for a considerable number of errors and received negative reviews from the gaming press. Critics targeted the overpowered races and classes, and Lawrence Schick noted in his 1991 book Heroic Worlds that many players regarded the new character classes as out of line with those in the Player's Handbook.
The errata were not incorporated into later printings of the manual. Kim Mohan published four pages of corrections in the November 1985 issue of Dragon magazine, but the corrections only appeared in a printed book when Wizards of the Coast released a premium edition with gilded pages on the 19th of February 2013.
The book introduced the barbarian (originally from Dragon issue 63), the cavalier (Dragon issue 72), and the thief-acrobat (Dragon issue 69). It also expanded and revised the druid and ranger classes.
The Unearthed Arcana digital series began in February 2015 on the official Dungeons and Dragons website, presenting work-in-progress playtest content for fifth edition. By August 2023, Jeremy Crawford reported the series had received more than 500,000 player surveys, and much of the material was developed into published books including Tasha's Cauldron of Everything in 2020.
The 2004 book, written by Andy Collins, Jesse Decker, David Noonan, and Rich Redman, was a 224-page supplement aimed at providing variant rules to change the game rather than simply adding new classes and spells. Designer Andy Collins stated that the default recipe for d20 products already looked like the original Unearthed Arcana, so the team chose a different approach.