It's Season 17 in Diablo 3, which started on May 17 because Blizzard are whimsical that way.
So, for whatever reason you want to level in Diablo 3 and you're not interested in slowing down to see the game.
Patch 2.6.5 will hit the PTR for a week starting on Friday, April 5th.
Diablo 3's patch 2.3 is launching this week.
So you've been persuaded to try Diablo 3 (maybe you read about the basics). You've loaded into the game, and now you're faced with the question of what game mode to play. There are many modes of play in Diablo 3 and some of the modes refer to what content you're playing rather than what difficulty.