Hey! And welcome to the forum!
I could be wrong on this as I'm not an expert in this field so this could be rubbish, but from what I understand hits is the number of file loads on the server. For example, if you had a page with text and an image (say index.htm and photo.jpg), when someone went to the index page, the image would be loaded as well so that would count as two hits. Now, the number of visitors is the number of requests for your site/page from different IP addresses i.e. the number of different people looking at your
website hosting.
And difference between registered members and visitors is nothing when it comes to bandwidth if they load the same files.
Hope that helps (and sorry if I'm wrong!)