There seemed to be some interest about learning more about this stuff, and while you all could do a google search, I thought I'd try my hand at explaining it. Then I'd also be around to answer questions and do follow up, which you can't get from a page on Wikipedia.
Ok, here we go. When you take pictures, there are different factors involved. There is ISO, aperture, and shutter speed. I'll have two more posts coming, about ISO and shutter speed, but this post is going to focus (pun intended!) on aperture.
Aperture is also referred to as F-stop. A lot of the time you'll see the number displayed with an F in front of it. e.g.- F11, F4.5, etc.
The aperture of a camera is the hole in the lens. The number tells you how big that hole is. Now here is where some people get confused, but I'll try to make it easy. The smaller the aperture number, the bigger the aperture. So an aperture of F4, the hole (aperture) is a lot bigger than at F11. I have an easy way to remember it which I'll explain here in a second.
A wide aperture (the smaller numbers) is going to produce what is called a shallow depth of field. What this means is the area that is going to be in focus, regarding to distance from you. Imagine that starting at your camera lens, there is a tape measure, and it stretches all the way to the horizon. Pretend you have your camera set at a low aperture and are focusing the camera on something three feet in front of you. With this wide aperture, what would be in focus is the 3 ft. mark on the tape measure, and maybe a little before and after that, depending on just how wide the aperture is. (When you get into macro photography and super wide apertures, the area of focus can be as small as only millimeters on that imaginary tape measure.) Anything else is going to be blurry.
The opposite of that is a small aperture. (the bigger numbers) That is going to make a very big "depth of field" The area in focus on your imaginary tape measure is going to be a lot bigger, and at a certain aperture setting, will be all the way to the horizon.
Remember I mentioned that trick to remembering? Here it is: The bigger the number, the further that will be in focus.
Now let's discuss bokeh. Bokeh is what happens when you are shooting with a wide aperture and have a shallow depth of field. It is when the background has that blurry and pretty look. It is also worth thinking about. If you are shooting with a wide aperture, just because the background won't be in focus, doesn't mean that it won't matter. Paying attention to the background can make the difference between an ok photo and a great one. For instance, if you are shooting with a tree in the background, move around to different angles. Sometimes one angle will make light pop through in an interesting way that will make the photo look more interesting than if the background were just the leaves.
I'm attaching two examples. Same flower, same camera settings, but a different background makes them two completely different pictures.
So there you go, aperture and bokeh and what they mean.
I'd be happy to answer any questions as well!