Unearthed Arcana refers to two different books.
Unearthed Arcana (AD&D)[]
The original Unearthed Arcana was an expansion to the Player's Handbook that introduced the barbarian and cavalier classes. It also introduced the concept of classes and sub-classes.
Unearthed Arcana (3rd edition)[]
The 2004 edition, which consists of almost entirely OGC, offered a host of optional rules and variants for the third edition rules.
Unearthed Arcana (web series)[]
The name has also been used for a series of web enhancements featuring optional or un-playtested rules.