Aperture is how big or small the iris or diaphragm of your camara opens. When you look through your lens as you press the button, you will see something like this.
Photos from
http://www.trustedreviews.com/images/article/inline/3290-aperturelarge.jpg
Basically, this is like the iris of your eye. It opens bigger to let more light in and smaller to have less light in.
Here's a sample and explanation from this website.
http://www.shortcourses.com/use/using1-9.html
Another thing the Aperture does is to increase or decrease the Depth of Field or DOF. The F-Stop is what indicates the Aperture size. So F/1.4 means the aperture is set at 1.4 which is a wide opening and F/8.0 means the aperture is set at 8.0 which is a smaller opening.
When you have a lower aperture setting, you have a shallow depth of field. If you have a smaller aperture setting, you have a longer depth of field.
Remember, the aperture size is the reverse of the F-Stop number. So when you have a big aperture size, meaning at the widest opening, the F-Stop is at the lowest number like F/1.4.
The smaller the aperture size the bigger the F-stop number like F/8.0.
Imagine your eyes, when you have your eyes wide open, you usually focus on the nearest object and the farther ones are usually blurry. We don't see this all the time because we unconsciously refocus our vision. But when you try to look at a very far object, you tend to squint your eyes to focus better.
That's what the aperture does. The smaller the aperture size, the more clear it is from front to back. The wider the aperture size, the more blurry the background.
Here's a sample
Photo from
http://karlgrupe.com/blog/wp-content/uploads/2008/10/depth-of-field.jpg
The photo on the left has a high F-stop or small aperture F/16. You can see that the sign and the green building behind is completely sharp.
The photo on the right as a low F-stop or wide aperture F/2.8. You can see the sign is sharp but the building is blurry.
Depending on how you plan to show your photos, you set the aperture to high or low. If you want to show a landscape with every details in sharp focus, you need to set your F-stop higher.
If you are shooting a portrait or a person and you don't want a distracting background, you set your F-stop lower to blur the background.
Making the background out of focus makes your subject the center of attention and will not distract from the overall photo.
For example, if you have a subject sitting in an outdoor restaurant with cars and people behind her, if you have a high F-stop, your photo will show everything behind and your subject may be lost in the crowd. But if you set your F-stop lower and make your subject sharp but the background cars and people are blurry, you focus the photo on your subject.
If you are taking a photo of a row of trees lined up towards the horizon and you want every tree to be sharp, you have to set your F-stop higher. Sometime like F/4 and up depending on your camera and the lighting conditions.
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