Overview Understanding of Theme Development?

Hello all. I have been reading through the development literature and am beginning to have an understanding of how WordPress is becoming a whole new entity within web development.

I work in several languages/dev environments and fortunately PHP, JQUERY (JS), and CSS are some of the elements included (which I understand to be THE tools used within WP)

https://www.levlane.com is a current site that I just wrapped up and am now in the next phase of optimizing.

My question is, as a 10yr+ web developer – could you give me the overview of how WP is being used and how themes are being developed?

I think that once I get the concept, then the rest will fall into place…as it has done with other languages/environments.

I am thinking that WP offers several native ‘pages’ such as search, comments, blog (list view), blog detail, etc and then WE have the option to modify them pages?

I am also thinking that there are native reserved functions and variables that WP provides and we simply tap into I.E., loops to query recordsets, page titles, header info, etc?

As reference, I have the default TWENTYTEN theme and a purchased ASPIRE theme. The answers to the above will help me to understand how much of what I see in the files are local to the theme author VS proprietary to WP…THEN I CAN GET STARTED BUILDING MY OWN WP THEMES AND PROJECTS! Yaaay!

Thanks!
Rich