Unearthed Arcana is the testing ground for new D&D rules--but at the dawn of TSR, UA was infamous for unleashing more variants than a season of Loki.
Unearthed Arcana is a long-standing tradition of D&D. Nowadays it's where you can get a glimpse of what WotC has planned for the future, whether you're looking at rules that will eventually make their way into the game in the form of new subclasses or races; or at wild experiments that break the game (but make like 15% of the people extremely happy). RIP Mystic.
But like many things that are popular in 5th Edition, there's a long long history there. And Unearthed Arcana has some especially long roots, stretching back all the way to the height of 1st Edition. In many ways, Unearthed Arcana embodies the wild and experimental feeling of early community-driven D&D. A big part of this has to do with the origins of Unearthed Arcana, which are tied inexorably to the community aspect of D&D. Because the rules that would be introduced in this infamous book were first run in the magazines that would help shape so much of D&D's future. In particular, in Dragon and Imagine magazines.
Bell of Lost Souls Staff Writer and DM, J.R. covers RPGs of all stripes and on occasion eats sandwiches. You can ask him about either at [email protected]