Aperture and how it effects focusing

Aperture is the term given to the blades within a lens which can be made smaller or larger, this not only changes the depth of field (a shallow or deep depth) but also the amount of light which goes through the lens.
Aperture besides being used to adjust the amount of light that goes through the camera it is also used to change the depth of field, but what is depth of field?

firstly depth of field has to do with the focus of the background, for example if you look at the top image in comparison to the bottom image, the background of the top image is blurred much more in comparison to that of the bottom picture. Shallow depth of field is where the background is in focus (or fairly in focus this was due to my bad focusing skills) whilst deep depth of field is where the background is mostly blurred and often not distinguishable. 

Normall a shallow depth of field would be used for something such as landscape photography whilst a deep depth of field might be used for something such as street photography where a smooth blurred background is often looked for to be in contrast with that, that is in focus.