Why should I avoid using frames on my Web site?
Because search engines spiders cannot read the contents of Web sites that use frames. A framed Web page consists of a home page — or frameset —, which contains HTML code that defines and links to the frames themselves. When visiting a page that is using frames, a search spider only sees the HTML code on the frameset page, not the primary page body contents, which are located in the frame elements. Thus, visiting spiders don't have any actual page content to work with and are therefore unlikely to give the page a high ranking.
An alternative to omitting frames is to add a Noframes tag to the master frameset page. The Noframes tag enables you to create an alternate no-frame version of a framed Web site, and when a search engine — or browser that cannot read frames — comes across a framed site, it will instead read or display the contents of this tag.