For over 10 years, a common question asked by our clients is what exactly is a frame site? Why is it we frown upon such a design and constantly tell our current and potential clients to redesign a frame site? Being seen on the search engines is the most important reason why.
Let's first address what a frame site is. A frame site allows you to display more than one html document or 'page' in one browser window. Many people like to link to sites but do not want their customers to leave theirs. So they create a frame of their site around the middle of a different web page. Using this on a very small basis can be OK as long as you don't base the entire website on this method.
The reason search engines typically cannot rank sites in this design are the mulitple frames that have to make up the page. They usually reach the first main frameset and cannot find any navigation or content.
Sometimes a portion of a frame site will be indexed in search engines. When this does occur, typically the main frame target or middle of the page is the only one that indexes. This causes a lost page; no navigation, no links, no logo. This is just as bad as no rankings at all.