Ever met someone who's breath smells like they literally just ate a shit sandwich? Imagine what their ass smells like.
Bilbo's timing is awful: Tom feels a sneeze coming on, so grabs his hankey - and the hobbit, too, by accident. He sneezes all over Bilbo before realizing what he has in his hand. Weirded out by the way Bilbo squirms, Tom tosses him on the ground. William asks him if he's a squirrel, and Bilbo answers, "I'm a burg - a hobbit." Tom calls him a "burglarhobbit" and the three trolls try and grab him. After a little bit of dodging, William picks Bilbo up by the feet and asks if there's any more little fellows around. Bilbo immediately says "no," but Tom thinks he's lying, and wants to hold his toes over the fire. Kili bursts out of the bushes alone, and attacks Tom with his sword. He demands the trolls drop the hobbit, so they throw Bilbo at him. That's when the rest of Thorin's Company attacks: wielding swords, hammers, axes, and spears.
I'm pretty sure Dori straight-up castrates Bert here.
In the chaos, Bilbo takes Tom's knife and cuts the knot keeping the ponies prisoner. The trolls notice what he's doing and grab him, holding him by the arms and legs. They threaten to rip his arms off unless the dwarves lay their weapons down. Following Thorin's lead, they begrudgingly do so, and all 14 members of Thorin's Company are captured.
The Book: One of the trolls says exactly the same line as the movie: "Mutton yesterday, mutton today, and blimey, if it don't look like mutton again tomorrer." The difference is, the complaint is valid this time: the trolls do not have horses to cook, nor had they just recently eaten a farmer's family (though William does make mention of Bert and Tom having had eaten an entire village and a half). Bilbo is unseen, and having discovered that its trolls at the campfire, has done his job and can report back to the dwarves. But he remembers reading that a truly exceptional burglar would not go back empty handed. To impress the others, he tries to pickpocket William as the trolls drank. William's pocket had a purse in it, but it was a magic purse, that spoke as it was touched: "'Ere, 'oo are you?" it squeaks, giving Bilbo away. Bilbo is immediately snatched by the neck and asked what he is. He answers: "Bilbo Baggins, a bur - a hobbit," and the trolls call him a "burrahobbit." They lift him by his toes and ask if there's any more of his sort hiding around. Bilbo says, "Yes, lots," then immediately says "No, none at all." He then pleads with the trolls, offering his services as a skilled cook if the trolls would only leave him alive. William wants to let the hobbit go, but Bert wants to hold his toes over the fire, and they have a fight over it.
Bilbo is dropped in the confusion, but struck by a wild swing from Bert and lay stunned. He watches the trolls fight for a short while, then Balin approaches the campfire. The trolls stopped fighting immediately and shoved a sack over Balin's head. Tom declares that there must be more dwarves, so they hide in the shadows. One by one, the dwarves approach, and one by one, a bag is placed over their heads. Bifur and Bombur had put up a fight when cornered. Thorin comes last, and Bilbo (who had escaped and hidden behind a tree) shouts that there are trolls; so Thorin leaps near the fire, grabs a flaming branch, and fights the trolls before a bag is placed over his head. All the dwarves are captured, and Bilbo is still hiding in the woods.
What difference does it make? Though this scene plays out very similar to the book - some lines are word-by-word identical - there are a number of very sizable changes involved. The first large change is the object Bilbo tries to steal. In the movie, he needs to steal the knife to cut the ponies free. In the book, there are no ponies for him to retrieve, and in fact his whole purpose there is to gather information, but he wants to steal something to impress the others. The magic talking purse he accidentally grabs is not in the movie at all. While the bag doesn't play any sort of vital role in the plot, it's one of the few magic items in The Hobbit; leaving out makes the world seem that much less magical. The method of capturing the dwarves is a large difference, too. The trolls are shown to be much smarter in the novel than they are in the film, by hiding and laying ambushes for each of the dwarves. The dwarves, comparatively, are shown to be much less intellligent in the book: they don't know what's at the campfire, and walk up one-by-one to get kidnapped. Only three of them put up any sort of fight. Bilbo's evasion of capture is important, as it changes how this conflict is resolved.
My Opinion: Some changes I like, some I don't. The talking purse always struck me as a little out-of-place in the novel, especially considering the role of magic, and magic items, in the Lord of the Rings saga as a whole. I'm personally alright with them removing it, but I can understand somebody disagreeing with this change - it's not really the kind of change to Tolkein's word Peter Jackson should be changing. The way the kidnapping of the dwarves was handled in the movie, though, is significantly better than the novel. I was honestly dreading this scene when I first saw the movie. It would get very tiring, very fast, to show a dwarf walk up, get a bag over his head, then another dwarf walks up and gets a bag over his head, etc etc etc. A battle like this is more climatic, interesting, and exciting - and it doesn't make any significant changes to the plot or characters involved.
Also: in the movie, all the dwarves, and Bilbo, are captured the same way. A sack is tied around them in such a way that their arms and legs are bound, but their heads are free. In the book, all the dwarves have a bag over their head, yet Bilbo managed to evade capture. This is another example of a minor change that will affect the conversation Bilbo has with Smaug; in addition to having been chosen for the lucky number, Bilbo "came from the end of a bag, but no bag went over [him]." Again, this loses meaning since he was no longer the only one of Thorin's Company to avoid getting a bag over his head (though, technically, none of them got a bag over their head, so...)