Its a Hack'n Slash/Dungeon Crawler , think torchlight or titanquest, rpg from a rts kind of pov you choose a class , a set of spells and nuke over some thousand of mobs , check some stuff on youtube : D
This is not to be rude or anything. But you really need to google some information before asking a question like that.
You need to be more specific on what you want to know, cause we can't break it all down
Even though the prior post did a decent job on describing the game - that's far from everything you need to know. So I would follow his advice and go to youtube etc.
That being said, you should definitely try the game out! I can't wait, and neither can you when you start reading about it and watching youtube videos about it! :-)
It is a multiplayer (optional) dungeon crawler where you choose a class, have a set of abilities, you follow a quest/storyline to complete tasks by moving through (semi-randomised) environments and dungeons, to get (mostly randomised) loot of varying qualities to upgrade your character. You get levels, points to spend, skills to unlock, crafting stuff you can use to gear up - and you progress through the game until you complete it. Once you complete it, you can go back to any point and replay/refarm gear, you can then turn the difficulty setting up (and up, and up) getting more awesome and rare loots and challenging yourself, and if that's not enough, you can play on hardcore mode which has different restrictions and penalties. You can choose to invite friends to play in your game and play together, or you can join a public game (4 players max) and there is optional limited PvP in an arena-style setting, or you can play the entire game in a solo setting.
D3 has an auction house which is new to the series, and controversially, has both an in-game currency and real-world currency options.
For the style of gameplay, there are loads of beta videos on youtube.
Not sure what else to add really, it's definitely worth giving it a go as it's with your annual pass, at least give it a playthrough once to see the storyline