Congratulations show watchers who aren't book readers! The time has come for you to be able to tell your pretentious book reading friends who've been trying to spoil the show for you to piss off, because the show is going to have to start spoiling the books.
Here's what show creators David Benioff and Dan Weiss have to say about it:
We've been talking to him for a while about this. The fact is we didn't want to catch up, obviously, we would hope that the books would come out ahead of the show. But at the same time George has his process and if it takes him 20 years to finish the series, that's what it should take him. I mean, he's writing, to my mind, the great epic of our, the great fantasy epic anyway of our time. We can't rush him and I wouldn't want to rush him. And at the same time we can't put the show on hiatus. We've got all these actors, wonderful cast, I mean I think the greatest cast ever assembled for a television show. And we can't take a two year break cause you know, these guys are in demand and they are going to go off and do other stuff. So we kind of have to keep pushing forward.
Luckily, we've been talking about this with George for a long time ever since we saw this could happen, and we know where things are heading. So we'll eventually, basically, meet up at pretty much the same place where George is going; there might be a few deviations along the route, but we're heading towards the same destination. I kind of wish there were some things that we didn't have to spoil, in terms of… for the books, but we're kind of stuck between a rock and a hard place. The show must go on [laughs]. Just came up with that. And that's what we're gonna do.
I think the thing that's kind of fun for George is the idea that he can still have surprises for people even once they've watched the show through to the conclusion. There are certain things that are going to happen in the books that are different from the show, and I think people who love the show and want more—want to know more about the characters, want to know more about the different characters who might not have made the cut for the show—will be able to turn to the books. So, that's where we stand.
TLDR? GRRM is taking a long time to finish this series, and while D&D respect that, the show must go on (no really, they made that joke). The show will be forced to spoil some important plot points from books yet to be written, but a lot of things will be different between the two versions, too. Just think of all those characters who aren't even in the show that are in the books; they'll need conclusions too!