Aperture and focusing

Hello.

I have a question on a relationship between aperture and focus.

I learned that a aperture is a measure of how much light is being absorbed into the camera.

So, maximized aperture, i.e. f/2.8, will have more lights compared to small aperture, i.e. f/22.

If an aperture is maximized or low f-number, then shouldn't be the picture all focused? 
(http://www.dpreview.com/learn/?/Guides/lens_buying_guide_01.htm)
Based on a picture of a purple flower, the picture is only focused on the flower itself from other leaves. Shouldn't be all leaves and flower to be focused in max aperture?
Or is it the depth of field that made the object sharply focused and others to be blurred?