Framesets come in and out of "fashion" but they do offer useful properties for a website. Most browsers allow framesets but it is possible that in future developements that this feature will not be supported.
Framesets allow you to display a number of HTML web pages in a single browser window. This can allow you to maintain some elements on you page without having to reload the element everytime the page is refreshed. This is good to display banners and headers as well as navigation menus.
To create a frameset you make the HTML pages you want to show first using the Doctype:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
This is placed before the <head> tag.
Then you create the frameset master page which defines the layout, page position, of each HTML page. This uses a different Doctype:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN" "http://www.w3.org/TR/html4/frameset.dtd">
The pages hold the content HTML and the Framest defines the layout of the whole page for the browser.