Crabbe and Goyle are Malfoy's sidekicks, together possessing the collective intelligence of a petrified tree stump. Basically, they are somewhat akin to casino bouncers.
In the first four books they don't actually do much, though. In fact, everything they do in the first four books can be summarised in about one sentence. Or maybe two: they follow Malfoy around, laugh at his jokes, and 'flex their muscles trollishly' when the situation demands it. And they crack their knuckles. Apart from that, they really don't do much at all - except maybe help to clear excess food.
JK Rowling describes Goyle as 'as mean as he is stupid,' but really he isn't that mean. Just stupid. Malfoy is the mean one, but since Malfoy has to think for both of them, I guess that makes Goyle mean as well. The same goes for Crabbe, who really isn't much different from Goyle. That's part of the reason I haven't bothered setting up separate sections for them. Think of it this way - you won't be far from the truth: Crabbe is Goyle is Crabbe.
Er, I guess there's not actually much else to say about Crabbe and Goyle. Goyle is supposed to be stupider than Crabbe, but it doesn't really show. Maybe it's because Crabbe and Goyle are never apart from Malfoy, and Malfoy talks enough for all three of them.
As a side note, Crabbe and Goyle's respective fathers are both Death Eaters (see end of fourth book). So I guess that places Crabbe and Goyle firmly on the 'bad' side. Still, they're too stupid to really be evil.