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An array of query vars are available for WordPress users or developers to utilise in order to query for particular types of content or to aid in theme and/or plugin design and functionality.
Public query vars are those available and usable via a direct URL query in the form of.
example.net/?var1=value1&var2=value2
Private query vars cannot be used in the URL, although WordPress will accept a query string with private query vars, the values will not be passed into the query and should be placed directly into the query. An example is given below.
query_posts('privatevar=myvalue');
See this list:
var $public_query_vars = array('m', 'p', 'posts', 'w', 'cat', 'withcomments', 'withoutcomments', 's', 'search', 'exact', 'sentence', 'debug', 'calendar', 'page', 'paged', 'more', 'tb', 'pb', 'author', 'order', 'orderby', 'year', 'monthnum', 'day', 'hour', 'minute', 'second', 'name', 'category_name', 'tag', 'feed', 'author_name', 'static', 'pagename', 'page_id', 'error', 'comments_popup', 'attachment', 'attachment_id', 'subpost', 'subpost_id', 'preview', 'robots', 'taxonomy', 'term', 'cpage', 'post_type');
var $private_query_vars = array('offset', 'posts_per_page', 'posts_per_archive_page', 'showposts', 'nopaging', 'post_type', 'post_status', 'category__in', 'category__not_in', 'category__and', 'tag__in', 'tag__not_in', 'tag__and', 'tag_slug__in', 'tag_slug__and', 'tag_id', 'post_mime_type', 'perm', 'comments_per_page');
via http://www.cleancode.co.nz/blog/731/post-data-wordpress-permalink-404
This list needs more work, if you have something to add go ahead.
