One (fairly) important difference between the book and the movie:
I'm still reading the book for the first time, but I literally just finished reading the most striking deviation from the movie so far; it might not amount to an enormous change to the overall plot, but it is very interesting (at least to me), and it was certainly an incredibly significant, influential, and life-changing event for at least one of the characters:
In the book, Unkar Plutt shows up at Maz' castle and threatens Rey. She's with BB-8, but no one else is around to help her. She pulls out her blaster and threatens to shoot Plutt, but he notices that the safety is still on, and snatches the gun from her hands. He smugly flips the safety off, and is presumably about to turn the gun on her and force her to leave with him.
Just then, an even bigger hand appears:
The upraised blaster vanished from Plutt’s hand, yanked away by a much bigger set of fingers.
Startled, Plutt looked back—and up, into the furry face of a deceptively calm Wookiee.
Not especially eloquent of Chewie, a relieved Rey thought, but it got the point across. Plutt wasn’t impressed. Noticing the bandaged shoulder, he poked at it with the same hand that had swiped Rey’s weapon.
“Half a Wookiee ain’t much to worry about.” He started to retreat into a fighting stance. “Not against all of me.” He lashed out.
Grabbing the thrusting arm, a roaring Chewbacca twisted and ripped it off at the shoulder, throwing the dismembered limb clear across the room. Looking down at himself, Plutt let out a scream of agony as his underlings hurriedly fell back.
The arm landed on a table where a group of four-armed, long-snouted Culisettos was gambling. With an annoyed huff, one of them picked up the amputated limb and absently tossed it aside, allowing the game to resume. Nearby, a small bipedal GA-97 droid who had been monitoring the pastime turned curiously to check the source of the excised limb.
- Alan Dean Foster's novelization of The Force Awakens
Of course, if you've seen the movie, you know that this entire scene is completely omitted, and we don't really see Unkar Plutt after Finn, Rey, and BB-8 escape Jakku. Although there is no reason to believe that Plutt will show up again in the series, and although his character is relatively unimportant to the larger story, this turn of events is remarkable for its coolness.
And of course, even though this passage probably won't affect our heroes in a serious way, and won't change the story much, it is presumably an extremely significant event in Plutt's eyes. After all, if you walk into a bar in good health and in possession of four limbs, and leave with only 3 limbs and a substantially lower amount of blood in your veins, you'll probably remember it for the rest of your life. He has joined Ponda Baba (the spider-face guy from the Mos Eisley Cantina) in the unhappy ranks of people from the galaxy far, far away who went to a bar, messed with the wrong person, and came out of the encounter with an arm missing. And I would argue that Plutt is worse off than Baba, because the latter was lucky enough to have his arm cleanly cut off with a lightsaber, which minimized the blood loss; Plutt, on the other hand (no pun intended), had his arm literally torn off by sheer, brute force. The fact that it was torn off at the shoulder probably means that restorative surgery will be more complicated and less likely to be entirely successful.
Why is this important?
One fairly significant plot point comes out of this scene:
Visual recognition ignited a small but very important internal sequence that concluded with the GA-97 sending out a compressed signal that was bounced around, coded, decoded, encrypted, and flashed out into deep space. Where it very soon was picked up, decoded, and decrypted, to become the impetus for an electronic shout of joy.
In the book, although not in the movie, the Resistance is actively seeking BB-8, and C-3PO is in charge of the operation. However, when BB-8 was sent out with Poe, 3PO didn't think to turn on the droid's long-range tracking device, so he is forced to alert all Resistance droids across the galaxy of the fact that BB-8 is MIA with some very important Resistance intelligence; any Resistance droid that sees BB-8 is ordered to report to headquarters at once.
When Chewbacca rips off Plutt's arm and tosses it across the room, one of those droids is present, and looks to see who is responsible for the dismemberment. Chewbacca is standing beside BB-8, so the Resistance droid sees BB-8 and sends a message to 3PO, letting headquarters know where BB-8 is. This is presumably why the Resistance showed up at just the right moment when the First Order attacked Takodana.
The other differences:
So far, the other deviations from the movie have been relatively trivial:
The spy who tells the First Order that BB-8 is on Takodana is wearing a dress instead of pants
Rey accidentally frees the rathtars by fiddling with a flow control panel rather than some fuses
Finn says a few things that appear to slightly contradict what he said in the movie (e.g., his experience as a stormtrooper is apparently much more extensive in the book than the movie)
A few scenes have been added, but they don't change the story in a significant way (e.g., we see Poe's hike through the desert, and learn exactly how he managed to get off of Jakku)
There is quite a bit of additional dialogue and exposition, but again, it doesn't alter the plot very much (e.g., we get glimpses into BB-8's thoughts and feelings; we see more of Ren's relations with his subordinates)