If you're wondering what on Earth the distinction is between descriptive and detailed, don't worry. We haven't decided to run amuck with synonyms. First, let's tackle descriptive. In terms of style, descriptive means that the book spends a lot of time telling us about things: how they look, what color they are, how they sound, what they resemble. Since the wizarding world is so alien to us, it makes sense that the novel spends a lot of time telling us all about it. Descriptive applies to characters too – we hear a lot about what certain characters look like. Check out the description of Lupin, when we first meet him:
Professor Lupin appeared to be holding a handful of flames. They illuminated his tired, gray face, but his eyes looked alert and wary. (5.161)
We learn a lot about Lupin here, before he even starts to speak much. We know that he looks tired and aged beyond his years, but that he's still "alert" and is presumably a good wizard, if that flame trick is any indication.
Overall, the novel's style uses descriptions to give us a vivid picture of Harry's world and the people in it. Which brings us to detailed.
Detailed here means that we get tiny tidbits and information that actually mean a lot to the novel's plot, to characterization, and especially to the novel's multiple mysteries – think of details as clues that a careful reader will notice. Let's check out another example with Lupin.
"But Sirius Black escaped from them," Harry said slowly. "He got away [...]"
Lupin's briefcase slipped from the desk; he had to stoop quickly to catch it.
"Yes," he said, straightening up. "Black must have found a way to fight them." (10.2.40-2)
There are a lot of tiny details in this scene that give us clues to Lupin's state of mind and to some of the larger mysteries going on in the novel. And it's worth noting that the style shifts when we get details like this – the sentences grow a bit shorter and are set off in their own paragraph. It can be easy to skim over a sentence like that if you're reading quickly, but in terms of style, the book also offsets the minor details and make them stand out to those who are paying attention.
Finally, we have fast-paced. This novel is a lot of things: fantasy, mystery, drama. But it's also an action/adventure tale in places and the style definitely reflects this. We get long action sequences with short sentences and/or clauses, lots of dialogue, and limited input from the narrator (just check out the sequence where Sirius the dog attacks and drags Ron into the Whomping Willow in Chapter 17). In action scenes, the narrator gives us a blow-by-blow of the action and the style is very quick and to-the-point.
We can contrast this with downtime passages, such as places where we get insight into Harry's state of mind (see Harry's nighttime musings in the infirmary at the start of Chapter 10). Here, the style shifts towards longer sentences and paragraphs, more descriptive terms, and little to no dialogue. Passages like this aren't too common in the book, though. Generally, the novel proceeds at a pretty fast pace and the style, with a tendency to favor shorter sentences and dialogue, reflects this pace.