There have actually been a whole bunch of differences between the books and the show in each season.
Examples:
Season 1:
- The scene between Robert and Cersei, talking about their marriage (one of my favs in the first season) never happened in the book.
- Ros isn't a character.
Season 2:
- Amory Lorch ends up being killed in the bear pit in the books, whereas in the show, Jaquen H'gar kills him.
- No appearance by Vargo Hoat (he's replaced by that guy from the Dreadfort who cuts of Jaime's hand).
- No appearance by Patchface or Jinglebell (the show does not suffer fools).
- Shae is never really in love with Tyrion, although he's in love with her.
Season 3:
- Robb's wife is completely different. (In the books, she's Jeyne Westerling, and she's still very much alive.)
- The Smalljon Umber gets beheaded at the Red Wedding. The Greatjon (who in the show had his fingers bitten off by Robb's wolf) survives and remains in captivity at The Twins with the Freys.
- Lady Stoneheart appears.
- The whole sequence at the Eyrie with Sansa and Littlefinger is DRAMATICALLY different. Littlefinger seems determined to CONTROL the Eyrie as his power base, rather than leaving and taking Sansa north (at least as of Book 3).
Season 4:
- This isn't exactly a "deviation" from the books, but the show starts to outpace the books and add in more scenes (e.g. much of what happens to Theon in this season doesn't appear in Book 4 at all). Actually, Theon's stuff might've started as early as Season 3.
- The timing of other events carries over from book 3 (e.g. the "Purple Wedding" where Joffrey dies). Also the sequence of certain events is a bit different (I think Jaime and Cersei have their tryst at the Sept of Baelor at a different point, and it's much more obviously consensual -- although it's still from Jaime's perspective -- than the controversial episode on the show).
Season 5:
- Tyrion's trip to Mereen is COMPLETELY different. He joins up with different people who don't appear at all in the books (Griff/Jon Connington, young Griff/Aegon VI?, a septa who may or may not be one of the Dayne sisters, etc.).
- Quentin Martell and his companions appear.
- Barristan Selmy is still very much alive, and ruling Mereen.
- Tyrion never meets: anyone at Mereen nor Jorah Mormont.
- Jorah Mormont is completely missing.
- All of Dany's stuff in Mereen that we see in the show in Season 4 is actually from Book 5.
- Brienne's journey is TOTALLY different (and kinda pointless).
- Book 5 contains all of Arya's training at the House of Black and White, which is also very different
And so on, and so forth. I'm sure I missed MANY points.
Put simply, the books and the show are two very similar but totally separate universes. I'd say at this point that the broad stroke of the plot points corresponds or each, and the "end states" also correspond, but the path between a major plot point and an end point may be different, and many of the plot points are changed to "fit" the show. Often with condensing characters and storylines.
This season is, as said, about 95% new to people. Much has been speculated that came to pass on the show, but we're past the books almost completely. I think the only stuff from the books at this point is some of Sam's journey to Oldtown, but if memory serves, his journey itself was different, too. Oh, there's another difference. "Baby Sam" is a pile of ashes because he was burned as a sacrifice by Melisandre after Mance Rayder's son was swapped out for him. Needless to say, Gilly isn't very happy.