As well as controlling how much light comes through the lens, the Aperture controls the Depth of Field (how much is of the image appears sharp). Adjusting the aperture can yield some very interesting results. A wide aperture value of around F/2.8 will result in smaller depth of field than an aperture value of F/32. You can see an example of this in the image below.

Aperture values are fractions. They are often writen as F/x. An aperture value of 2 is actually 1/2, an aperture value of 22 is actually 1/22. An aperture of F/2 is wider (larger) than F/22. Just like a piece of pie, 1/2 is a larger than 1/4.

Choosing a smaller aperture (higher F number) may not always result in a sharper image. As the aperture size decreases you may find that the overall image loses clarity. This is due to the occurance of diffraction particularly noticable with smaller apertures. You can see in the below image the bottom chain at F/32 is not as sharp as it is at F/16.