I know its not a big deal. And I'm sorry for the fuss.
Grommash "Grom" Hellscream is the former leader of the Warsong clan, he had many victories to his name in the conflict on Draenor, the First and Third wars, notable ones including besting Nobundo at Shattarath and Cenarius at the foot of Hyjal among many others. He died giving his life to kill Mannoroth and end the corruption over his people once and for all. In the upcoming WoD expansion, an alternative version of him will be one of the Warlords.
Garrosh Hellscream is his son, a sickly whiny bitch who grew up on Draenor during the three wars, brought to Azeroth during the end of the Burning Crusade/start of the Litch Kings rise to power. He subsequently earned a place in the horde, eventually being named Warcheif leading the horde to the current situation it is in, and it is he that will be going back in time to cause the WoD crisis.
They are not the same person, stop using their names interchangeably.
I know alot of people will be like "duuuhhh" - but I'm just tired of people mixing up their names is all.
Again, sorry for the fuss.