How can I use resized images?

When I upload an image to my media, it is automatically converted into a few different sizes as defined by my functions (if i’m not mistaken).

I see other wordpress sites use these images interchangeably.

For example, a site may have in their sidebar a list of relating articles, though, instead of loading the articles featured image, it will load the same featured image with the converted “-000×000.jpg” size.

So for example, if I have an image that is “images/whatever.jpg”, I want to use that image as my featured image, but in another area of my site, when referring to relating articles or anything else etc, I want to pull the same images except in its smaller size such as “images/whatever-400×500.jpg”

I see it used all the time but I am not sure how to do this?

Is there a quick wordpress php script where I can select between the different converted file sizes?