Being 4 month away from the World of Warcraft: Legion launch, Blizzard has started the game's Beta test and released colossal patch notes. The current Beta test is a kind of the Closed Beta, although the developers refer to it as just Beta test. In this official announcement, you can find the main information about the test. As of now, only invited players can access Legion's servers.
Other than that, Blizzard has released huge patch notes for Legion Beta. You can read them here.