The basic idea, from what I've read, is that Garrosh escapes prison in Pandaria with help from rogue members of the Bronze Dragonflight (who we help get their stuff together on the Timeless Isles). The dragons seem to think that the Alliance and Horde are not strong enough to fight the Burning Legion that should be coming back...someday. So they send Garrosh back in time to help his father forge an untainted Iron Horde that uses "modern" technology to take over Draenor. This creates an alternate Dreanor from 30 or so years in the past.
They modify the Black Portal that was under construction to instead go to modern day Azeroth (so older, now alternate Azeroth is not a thing in the expansion). Thus we won't be sent to put things back into place, as it is too late for that, plus the changes don't effect Azeroth, not Outland's past. The Alliance and Horde have to go stop the Iron Horde, because they are coming to take over present day Azeroth with modern Horde weapons on untainted orcs with a full population behind them. That is more than the old Horde could do verses the Humans and later Alliance back in the Warcraft 1 and 2 days.
The Alliance and Horde stop the first invasion at the Dark Portal (now gone red instead of fel green), then have to send a crack (suicide) team to destroy or disrupt the portal to prevent more invasions that the Azeroth powers just don't have the power to stop after all the violence and cataclysms since the Third War decimated so much of the population centers.
The Garrison is to be your base of operations for the campaign you (player name) are running in Draenor against the Iron Horde. You might be a small part of the larger force, but this part of the army is yours. It will likely be customizable based on your character's race.
Also it looks like Leeroy Jenkins is now a canon character...who died in Blackrock Mountain.