This article is an introduction to the "brother and sister" Web Service protocols named Search/Retrieve Web Service (SRW) and Search/Retrieve URL Service (SRU) with an emphasis on the later.
"SRU (Search/Retrieve via URL) is a standard search protocol for Internet search queries, utilizing CQL (Common Query Language), a standard query syntax for representing queries."
"SRW/U sends a single URL to a remote computer and the remote computer sends back an XML stream. (...) since Web browsers are able to "transform" XML into HTML with a built-in technology called XSLT, the search results are displayed to the user."