Frames are a method of dividing the browser window into smaller sub-windows, each displaying a different HTML document.
Frames break the unified model of the Web.
With frames, the user's view of information on the screen is now determined by a sequence of navigation actions rather than a single navigation action. Navigation doesn't work with frames because the unit of navigation is different from the unit of view. If users bookmark in their browser, they may not get the same view back when they follow the bookmark. URL's stop working. Screen size will affect what the user sees.