WHAT IS AN F-STOP?
I've been asked this question a lot from people who are just starting out. I'm going to give you the most simple explanation of an f-stop that I can think of.
No matter what lens you have, it has an opening that can be made wider, or narrower. This opening is called your lenses f-stop (or aperture). It is designed to let a certain amount of light in to your camera. The lower the f-stop that your lens can go, the wider the opening will be, which allows more light in to your cameras sensor. These numbers often go as low as f/1.4, f/1.8, or f/2.8 which is great for shooting in low light situations. By being able to utilize these lower f-stops you can achieve faster shutter speeds, and not have to rely on boosting your cameras ISO (sensor sensitivity).
Now, In conjunction with letting light in to the camera the opening also plays another huge role. It creates depth of field. Depth of field is essentially the area that is in focus. The lower the f-stop, the shallower the depth of field, which means less of the image is in focus (from front to back) which really helps to separate your subject from the background. Here is an example. If you are taking a close head shot using an 85mm f/1.8 lens and focus on your subjects eye, the background will melt away almost instantly. The eye will be in focus, but everything from the ears and beyond will become blurry - very cool.
NOTE: Be very careful if you are shooting at lower f-stops. Because the depth of field is so shallow, the slightest movement can cause your photo to be out of focus. Try not to focus and recompose, but rather get your focus point where it needs to be and try locking in your focus a few times to make sure you've got the shot.
The higher you set your f-stop (something like f/8, f/11, f/16) the more of the image will be in focus. This is great when you are taking landscape photography, group photos, or using flash in a studio.
There is a lot that goes in to using your f-stop properly, and takes some practice, but knowing what it is, and how it works is half the battle.
The f-stop is your lenses aperture. The lower the number, the more light is let in to your camera. With a lower number, you can also get a really blurry background.