To teach people what to do after they have finished their novel.
So, you've spent weeks, months, maybe even years writing your novel. You finally finished, your main character got the girl/guy, or your main characters got each other, or your characters never had a love interest in the first place and nobody got anybody. Your antagonist is reformed, dead, or sent to some inescapable universe, or somehow is no longer a problem. You've gotten all of your flowery description typed, and you're finally ready to write a query letter. You'll send it in to an agent, who will weep with the beauty of your prose. You will get a great publishing contract, and all will be well with the world, right?
No. Not even close. Before you write that letter, before you even start looking at agents, you need to edit. I don't mean run it through spell check. This is the sort of editing that will chop thousands of words off your novel, but you know what? It's all worth it in the end. So come on, this is how you make sure you've got it right.
The first thing that you should do after you've finished your novel. You should wait.
Have you ever read anything that was published that had things appearing at random, buildings popping up in empty fields without being built, etc.? No! So why should your novel be any different?
When you're typing up your manuscript, you'll make little mistakes that are pretty obviously wrong.
There are a lot of weird specifics you need to follow when writing dialog. Here's the basic ones.
Source: http://evelynjanelle.blogspot.com/2010/10/all-about-dialogue.html, dictionary.com
Here's something else useful. I've never seen anything more annoying than people using em dashes and semicolons as commas.
Source: http://evelynjanelle.blogspot.com/2010/11/dashes-ellipses-and-exclamation-points.html, dictionary.com
Some other things that you should look for.