http://stevemccurry.com/galleries/portraits

I have been looking at Steve McCurry's pictures a lot.
Although a picture may have been taken many years ago, he can often say what the 'exposure' was, for example f2.8 at 1/8 sec at 400iso.
This picture has very focussed eyes, but the background is soft and unfocussed.
So this tells me that the aperture was wide (possibly f2.8)
We talk about this from day 1, and as photographers we never stop paying attention to it.

This is a useful website that explains almost everything you need to know about how digital cameras work.
You might be a bit confused after a couple of days of technical jargon
So, follow this link:
http://www.shortcourses.com/use/using1-6.html

Read the words and watch the animations. Then try to explain in your own words, the meaning of 'exposure' and how shutter and aperture effects it.