To understand WordPress we'll need to understand its terminology and, perhaps, its history. It started out as a blogging framework.
The Codex History page states:
"WordPress was born out of a desire for an elegant, well-architectured personal publishing system built on PHP and MySQL and licensed under the GPL. It is the official successor of b2/cafelog. WordPress is modern software, but its roots and development go back to 2001. It is a mature and stable product. We hope that by focusing on user experience and web standards we can create a tool different from anything else out there.
2001 - b2 cafelog launched by Michel Valdrighi.
2003 - Matt Mullenweg and Mike Little fork b2 and create WordPress."
So, to understand WordPress, we need to come to an understanding of the word "post".
post - It is a little confusing at first because this word is used in 2 ways: 1. as anything that is published. or 2. as a format type (you may have a post type, a page type, or any number of custom types).
template - a php file that will be used to display a certain type of file. For example, if you just stuck to the two basic types: post or page, you might have one template for post data and another for page data.
tag - a tag may be thought of as an attribute. The Codex states:
Unlike Categories, Tags have no hierarchy so there is no Parent->Child relationship like that of Categories. But like Categories, Tag names must be unique.
category - a category is a grouping, or, in mathematical terms, a set. Categories are like a book's table of contents, while tags represent the index.
the loop - the loop is where the code gets and displays data from the database. Underlying everything is the mySQL database. This is where the posts, pages, and custom pages live -- i.e., in the database.