Theme.fm has a series of three articles on how WordPress boots up, how it moves through the process as a visitor lands on their first page on your site and a step-by-step look at the WordPress Loop.

1. WordPress Internals: How WordPress Boots Up
2. WordPress Internals: How WordPress Boots Up Part 2
3. WordPress Internals: How WordPress Boots Up Part 3 – Theme.fm

As a visitor lands on a web page on a WordPress site, the browser begins the process of loading the visitor request to view a specific page. On that web page, there are codes that instruct WordPress to pull information from the site design files called WordPress Theme template files, from WordPress Plugins, from the database, and other code requests to literally build a viewable page instantly for the visitor. These articles dig deeply into how WordPress code works with all the various programs and options to generate that page.