Key Takeaways:

WP_Query is a powerful tool that allows you to fetch custom "bundles" of posts from the WordPress database—based on criteria you specify—using relatively simple PHP syntax rather than complex MySQL queries.
As WP_Query is a PHP class, every use of it begins by creating a new WP_Query object. This object takes a single parameter: an associative array that defines the post bundle it will fetch from the database.
When a WP_Query object has fetched its bundle of posts, this bundle is saved as an associative array. This array can be looped through almost identically to WordPress's default Loop (which, in fact, loops through WordPress's default WP_Query instance). This similarity means, for example, that WordPress template tags like the_title() are available when looping through a custom WP_Query object's fetched posts.

WP_Query is one of the most powerful APIs in WordPress developmen...