Spelling and grammar are absolutely integral to the creation of a functional work of literature. It is imperative that you check over your work for spelling and grammar goofs, because they detract from the story you are trying to tell. Way to state the obvious, right? One would assume this to be obvious, yet a startlingly high number of aspiring writers will present drafts that are so riddled with spelling and grammar errors that they become painful to read. However, more often then not it's a simple case of a few common errors. Common, but still the sort of thing that will make an editor cringe and hurt the readability of your story. For example:

To, Two, and Too

To - "To" serves a couple of functions. It can act as a preposition when it proceeds a noun ("Give that cookie to your sister."), or indicate an infinitive when it comes before a verb ("Don't you want to go skiing?").

Two - "Two" is the number 2 ("You can have two helpings, but not a third.")

Too - As with to, "too" can be used multiple ways. I can indicate an excess ("That show is too late at night, you have school in the morning.") or it can stand in place of "also" ("Shaun wants a Nintendo for Christmas too.")

Run-on Sentences - Punctuation is always good, but you should avoid using an excess of commas where a period would work a whole lot better. It is perfectly fine to have a compound sentence that joins several related ideas, but when you have too many dissimilar ideas together things start getting confusing. Let's take a look at a classic run-on.

"Larry didn't have any idea why he was tied up and he couldn't help wondering what his captors wanted from him when a loud, jingling started off to his right and his thoughts were interrupted..."

This may seem like a dramatic example, but believe me when I say that I've seen worse. Now, let's divide this up into a few sentences and see how it sounds.

"Larry didn't have any idea why he was tied up. What did his captors want from him? His thoughts were suddenly interrupted by a loud jingling noise to his right..."

From one sentence to three; see how much better that sounds?

The Big One - Using spell-check as a crutch. Almost all writing programs have spelling and even grammar check functions. These are very useful tools, but they are not foolproof. If a word is spelled correctly, and is correct grammatically, the spellchecker will ignore it. This can become a problem if it's not the word you were trying to write. Typos happen to the best of us, and it's ALWAYS a good idea to go back and read over your work. If you can, you should also get someone else to read it, because often you may not notice your own errors when reading because you know what it's meant to say and you're really only skimming.