My Warcraft Confusion

I really enjoy getting into the Warcraft lore, but the Warlords of Draenor expansion confused the hell out of me.  Of course I understand the expansion went back to an alternate reality of past events but I thought Blizzard specifically said this was an alternate timeline that does not affect our current world.  My guild has basically dissolved so I also didn’t get into any of the dungeons or raids that might fill in the blanks for me. 

Now with Legion coming out at the end of this month I see many teaser trailers with Gul’dan.  I’m thinking what? But Gul’dan’s dead in our timeline.  Is Legion also in the past timeline?  Why is Gul'dan still a part of Legion?

Also Vol’jin is the Horde Warchief.  Isn’t Legion going to be the first time he really needs to prove what kind of Warchief he can be? Or was that also something I missed in WoD?  I mean WoD was an alternate timeline so Vol'jin technically hasn't done anything "Thrall-like" yet correct?

Like I said I’m very confused.  Any chance someone or is there a good YouTube video can clear this up for me?

Thanks

Chad