Aperture is the size of opening in the lens. This controls how much light gets through to the sensor, as well as which parts of the photo are in focus. Aperture is part of the exposure triangle, along with shutter speed and ISO.
The aperture setting is often called the f-stop. The f-stop is an inverse number, like f/2.8 or f/11. This means a smaller number on the bottom actually results in a larger aperture value. Although confusing, once you remember that it is the inversed, it becomes easier to remember.
A larger aperture like f/2.8 will result in a shallow depth of field 
. This means only one area of the your photo will be in focus. For instance, a person’s face will be in clear focus, but the background will be blurred. With a small aperture like f/11, more or all of your image will be in focus. This setting is good for taking landscape photos, etc.
Aperture Explained 
http://digital-photography-school.com/aperture
Aperture and Shutter Speeds Explained
http://www.facethelight.com/aperture.php
Understanding Depth of Field
http://www.cambridgeincolour.com/tutorials/depth-of-field.htm
Exposure, Shutter Speed, and Aperture [Video]
http://www.labnol.org/software/tutorials/camera-shutter-speed-and-aperture-explained
F Stops [Video]
http://www.youtube.com/watch?v=vZdex71oOGY&feature=fvsr