Sites used to be structured with frames in the old days of web building, especially when the sites were doing more than just showing one page at a time. That changed out of necessity, as DHTML took hold and the limitations of frames became more evident.
Frames allow a developer to divide an application page into named sections that can still interact, but never overflow into one another. This has its advantages and disadvantages, as you can well imagine. On the one hand, it allows for easy layout from a development point of view. On the other hand, it is hard to create dynamic content that can interact anywhere on the page, because anything dynamic is constrained to its own frame.
If you decide to use frames, the XHTML 1.0 Frameset document type definition (DTD) is available, as is the HTML 4.01 Frameset DTD. Use whichever you like, but remember, the Web deals with XML a great ...