When you write, you need to know how many words to aim for. Not only does it help you set goals and pacing, if you seek to publish this will help. Agents and editors abide by these informal guidelines of word count, and readers subconsciously judge books by size, aka word count. 

For your first attempt at writing a first draft, aim for the middle of each range. Remember good editing will make it shorter and tighter, and going for the middle means you often end up towards the low end, but still within range. Publishers are there to make money, the less your word count the lower their printing costs. they never mind you being on the low end or slightly under, but they hate when you go over.

Don't ever go over on a first novel, that's one of the top five reasons a manuscript is rejected. Ever notice some of your favorite authors who write a series have the first four books are slim at 90,000 words, then suddenly book five is 175,000 and looks weird on your shelf? Also notice how the story line begins to lag there? Don't be a douche and do this. If you write a series, keep them all the same. It's hard work, but you can do it. Now, with that in mind, here are the suggested word count ranges for fiction:

Young Adult: 75,000-90,000
Young Adult Paranormal: 75,000-100,000

Historical or Paranormal Romance: 85,000-100,000
Category (Harlequin) Romance: 55,000-75,000
Modern Romance: 75,000-100,000

Genre Mystery: 65,000-90,000
General Fiction Mystery: 80,000-100,000
Thriller: 80,000-100,000
 
Horror: 80,000 - 100,000

Western: don't even bother, nobody will buy them

Epic Sci-Fi / Space Operas: 90,000-120,000
All Other Sci-Fi: 90,000-100,000

Now that being said, remember some editors can vary. Ideally aim for these goals to keep it tight and marketable. If you go over and you get a publishing contract, you have to edit it again. For example if you have a modern romance and the contract says it must be 90,000 words and yours is 130,000 you will have to shave off 40,000 before you can sign the contract.

So keep it short, tight, and simple, people!