Why are HTML frames bad?

There are various usability and accessibility issues with frames:

1. link can open in the frame it is enclosed in (e.g. a side pane);
2. can break the forward/backward navigation;
3. difficult to bookmark;
4. are not easily searchable (likely to see the content in Google, etc.);
5. break on browsers like Lynx, that are console/terminal based;
6. difficult to size properly (e.g. consuming height on widescreen monitors for banner frames);
7. can break with screen readers and magnifiers (for blind users and visual impaired users);

See http://www.angelfire.com/super/badwebs/ for an example of what not to do.

Depending on what you want to do, most things done with frames can be done with CSS. CSS stylesheets are compatible with all MODERN browsers, meaning your website will look the same if using firefox, chrome, or IE 7(with some tweaks). Also backward-compatibility is not a concern as users can view the content even with CSS off (where as a website using frames and without a frameless version of the site will be useless to a user with an old browser), it just won't be as stylized. It's also quite easy to learn, and once you get the hang of it you'll wonder why you haven't learned it in the first place.