WordPress Query Vars
Query vars define a query for WordPress posts.
When ugly permalinks are enabled, query variables can be seen in the URL. For example, in the URL http://example.com/?p=1 the p query var is set to 1, which will display the single post with an ID of 1.
Relationship to WP_Query
Query vars are fed into WP_Query, WordPress' post querying API.
List of Query Vars
Public vs. Private query vars
Public query vars can be used in the URL querystring. Private query vars cannot.
Private query vars can only be used when creating a query in PHP. For example,
<?php $query = new WP_Query(array( post__in => array(3, 7) ));
works, but visiting http://example.com/?post__in=3,7 would not work.