You play as an inquisitor and when the game is done you will have a entire subsector with heretics to subdue.
There will be a single player mission based option, random generated missions and investigations ( mini campaigns )
You will have your own fortress that you can customize and use it in a pvp like style.

There will be content after launch that shapes the subsector in terms of missions, threats and rewards.
(  Insurgence of nurgle worshippers are spreading if enough players work together to stop it it wont spread and if not nurgle will be stronger in that section of the subsector )

There will be three classes, but for now there is only one named the crusader.
He has access to weapons like grav guns, meltas, power swords, grav guns, autoguns and more.

For now this is in an early alpha state and while it is enjoyable it is light on content.
It is in my eyes a very raw diamond and we are here to temper it in the fire and make it the best 40k game ever.

Hope this answer helped! 

When you ask for Gameplay, let me post some of my own videos then;

A video about the Road Map (when to expect the things Vikingvesk talked about) - presented with some gameplay of the Melee Crusader in the background:


And here is a video with some ranged gameplay, featuring the brand new Grav Gun and Grenade Launcher - on a Ground mission :)


And lastly, but not least, my good buddy AirsickHydra is doing a lot of videos on the Lore of the game world and other rant videos which is definitely a watch worthy;