What's an F/Stop?

So, I have this photography hobby, that I want to turn more into a money making hobby.
Now, I'm no expert at mine or anyone elses camera.
But I want to be.
I know enough to get a pretty great picture, but I want to understand what I'm doing more, and really learn the how's and why's better.
I was hoping you all would join in with a discussion of sorts as we go through the settings on our cameras.
I know a lot of you have SLR's like I do, and we are all probably on different levels of understanding.
I think we could all benefit from everyone elses knowledge.
So let's get started.

Today I want to talk about APERTURE, because it falls in the most basic, and necessary things to know category.

This is what I (think I) know.

The size of your aperture (or f/stop) controls how much of your photograph is in focus.

When you change your f/stop, it changes the opening on your lens.

One thing to remember about f/stops and aperture is the higher the number the smaller the opening, and vice versa.

The smaller the opening on your lens, LESS light is let in, which allows more of your photograph to be in focus.

Here is an example of a photo of a pine cone I just took with my f stop set at 25.
Which is a very high f/stop, and considered a closed aperture. Notice how you can see clear to the back of my backyard, (which is very large, you can see the trees, and the stuff in our neighbors backyard).

The next photo I changed my f/stop to 14, which is still a high f/stop, but it opened the aperture up a little bit, and I lost some of that focus that was in the backyard.The next photo I took at an f/stop of 4.5
The aperture is WIDER and therefore let more light in to the camera, so LESS of my photo is in focus and gives me that nice blurred background.For taking pictures of people and they are all on the same plane you probably want to set your f-stop at a lower aperture, then the noise of the background is blurred and all of the focus will be on your subject.
If they are all over the place and on different planes then you want to set your f-stop to a higher aperture, so that they will all be in focus, but remember so will most of, or all of your background.
Unless you are trying to only focus on the people in the front, then you will want to adjust your f/stop accordingly.

So if you have never shot with your camera out of automatic, then I challenge you to shoot in your aperture priority (usually the Av or A symbol on your dial) mode for the next few days. Try to figure out if a higher or lower f/stop is required for the ideal photo you are trying to capture.
Share your examples on your own blogs, and link them in the comments.

For those of you who know more about aperture, please comment and let us learn more from you! Correct me on my mistakes, I'm sure I've made them, and let me know what you have discovered!

Next time, I want to discuss ISO (shutter speed and film speed), and how to adjust it with the aperture to get the best results.