A sequence of codes in a Uniform Resource Identifier (URI) that a web page uses to determine what dynamic data to display. The query string in a URI comes after an initial question mark, and may contain several parameters separated by ampersands. WordPress uses query strings to indicate criteria to search for specific posts or sets of posts in the database. The use of query strings is generally believed to impede the indexing of dynamic pages by search engines. For this reason, it is often desirable to use a method such as mod_rewrite to reduce exposure of query strings to search engines and other site visitors.

Reference http://codex.wordpress.org/Glossary