First sentence: "Robert and Allison consummated their new marriage in the powder room of the house."
Second sentence: "As soon as they said 'I do', Bobby and Ally fucked each others brains out in the bathroom."
Ready to move on? You sure? Okay.
They're both the same sentence in that they both relate the same information -- a newlywed couple having sex. What separates them is tone. The first sentence is pretty G-rated, while the second is something you hear for a more adult audience.
Tone makes for a massive part of the writing experience. The tone of a piece can change from draft to draft and project to project, but with enough time, a writer's use of tone will cultivate to such an extent that a unique style is born identified with that particular writer.
What goes into tone? A large part of it has to do with the mood you're trying to convey to the read about the subject you're writing, and what goes into this mood is a combination of word choice, imagery, grammar and word order, detail or lack of it, and sentence length.
Let's look at word choice. In the two sentences, I have the words "consummate" and "fuck". Both mean the same thing because they relate to sex. Which word would you likely see in a Jane Austen story? Actually, you could see both because they have their origins in the early-1500's, but "fuck" is considered vulgar rather than "consummate", and because vulgarity relates to a more common social class (vulgarity and obscenity are not the same thing, mind you), it can give a piece more range with prospective readers.
Word choice also has an impact on imagery. Now, while "fuck" is a vulgar word, it's also an obscene one. When I hear about a couple consummating their marriage, I think of peaches-and-cream lovemaking, that really gentle and romantic variety. When I hear that Bobby and Ally are fucking, I imagine that they're going at it like animals. This is an example of how word choice alters the mental image a reader has when exposed to a piece.
Grammar and word order are, I believe, matters left to writers on a case by case basis. "Will cut through the ham with a sharp knife." sounds better than, "With a sharp knife, Will cut through the ham." I think Stephen King had a similar example in On Writing. My rule of thumb is this: the most important piece of information in a sentence ought to come first. What we need to know in this sentence is that Will is cutting a ham. The knife and its sharpness are minor details. On the other hand, if this was a scene in an operating room -- "With the scalpel, Will made an incision just above the kneecap." -- I would put the prepositional phrase first because, while we generally know what goes on in an operating room, the scalpel helps sharpen and clarify the scene for us.
Detail or lack of detail. Again, I prefer to leave this up to the writer. It depends on how much you'd like to leave to the reader's imagination. Going back to the sentences about Robert and Allison, we don't need to go into detail about how their having sex because that's something best left to the imagination unless we were reading erotica (I'll deal with erotica and sex scenes another time, you horny bastards).
Finally, there's sentence length. Cormac McCarthy's The Road is an interesting blend of short and long sentences. The dialogue between the man and the boy is short and to the point, and each one says only a handful of words at a time. McCarthy allows longer sentences in his description of things, but even then he often chops his sentences up. Here's a passage as an example:
"Beyond a crossroads in that wilderness they began to come upon the possessions of travelers abandoned in the road years ago. Boxes and bags. Everything melted and black. Old plastic suitcases curled shapeless in the heat."
McCarthy could have merged all of these sentences into one, but by breaking them up, he helps to relate the tone of The Road to the reader in a subconscious way, a tone that tells the reader that time is very short for the man and the boy, that the world is slowly dying, and that what we are really left with are just flashes of the past.