Portfolios Need to be Explained

I have a pretty good understanding of WordPress (posts, pages, categories, etc.), however, I am struggling with getting an understanding of how a Portfolio Plug-In works! [bobgriner.com]

I’m struggling to understand if a portfolio is a [post as a collections of images], a [virtual collection of images] that are grouped/displayed dependent upon categories/tags/filters, or [pseudo-posts] that must have short code placed into pages or posts to become visible?! I’m failing to understand the relationship between post, page, portfolio, and gallery.

I have purchased a beautiful, and complex, portfolio plug in [Viba Portfolio] to reside in my Graphene Theme, but my ignorance of the concept has prevented me from making anything post-able! My intent is to make a copy of my Graphene Theme, place it on a subdomain [work.bobgriner.com] and have nothing more than my work history Portfolio there. I would share that specific subdomain when applying for jobs, and it would not be found by going to my personal web site.

I have searched the web over and over to try to find a [Portfolios for Dummies] explanation, but just can’t find anything that breaks it down in an understandable way for me.

Thanks to the compassionate person who takes the time to explain this to me ??