Active and passive voice are something that, in general, teachers are much more concerned about than readers. However, it's a good to know the rules and to keep them in mind—it'll impress the hell out of teachers if they haven't taught it yet, and at some point, everyone runs into a college professor who gets pointlessly bitchy about passive voice in papers. Everyone.

I brought up a couple examples of active and passive in the tricky words section because it's something I find excruciatingly difficult. But basically, it goes like this: your writing will have a larger impact on the reader if it's direct. To keep it direct, actions should be preformed in the most direct way possible.

For example, David slammed the window shut. Okay, the action there is the window shutting. David performs the action. This is very obvious. This is the good way to write the sentence.

The bad way would be as follows: The window was slammed shut by David. Because, again, the action is shutting and David is the one who performs the action, but for some reason, it's the window that's being emphasized. There's an extra verb, was, thrown into the mix. This makes the sentence less direct, and so the writing is weaker. This comes into play much more when you're writing an argument (which is why professors tend to be more stickler-y about it), but it's worth it to keep in mind.

The tip that professors give is to try and use the verb, "to be," as rarely as possible. Usually, if you have a variation of it in the sentence (such as "was," in the previous example,) it implies something has gone passive, like David's opening the window. Obviously, this isn't always an indicator (such as that "isn't," I just used) but if you often use "was," with another verb, you should reconsider and see what you can do to make the sentence more active.

This isn't a big thing, and passive voice can certainly be used to create a very specific, dreamy or far-away tone. But it's worth it to keep in mind, because too much passive voice can give you that kind of tone unintentionally; and if that's not what you want, it can make a story read awkwardly and bore the readers. It's a classic case of knowing the rule so that you can break it.