The appearance of "no digital signature" merely means that the file doesn't include a "registered" name in the "Properties" of the file. While it's rather rare for malware infected files or pirated copiies of files to have a signature, many "fringe distributors" (like Adobe and HP) quite often don't include one, so it's necessary to guess whether you're downloading something authentic and good. Many times it is necessary to ignore the warning and charge ahead If you want to get anything at all accomplished.

Another "warning" that pops up occasionally is that a website has "no certificate of authenticity." This means that the site as a whole has not "registered with some proper authority." As with the "signature" this indicates that you should "pause and think" in case there might be a problem with what's about to happen; but in most cases any of us are likely to encounter either can be ignored for "recognizable" sites and programs.

So far as I know (I haven't checked lately) mudcat has never displayed a "certificate" and I don't worry about coming here. Whether you get a notice of either kind of certification depends a lot on what "whims and hallucinations" went through the few functional brain cells of whoever wrote the last update of your browser or AV suite, although sometimes the warnings are actually helpful.

A "digitally signed" browser add-on (sometimes called a "bho" for "browser helper object") that you add in recent versions of IE usually goes into a cache of "trusted add-ons." An unsigned add-on goes in a separate cache. The "functional difference" is that you can delete an add-on from the unsigned cache using Tools (Internet Options) in IE. You can disable, but you can't delete a "trusted add-on" using just the Tools in IE. This difference may have existed in older versions, but it's specifically spelled out for IE8 and IE9, and possibly as early as IE6(?) although I haven't looked back that far. This difference shouldn't affect any use of the browser or be a concern to the users.

I haven't seen any similar differences in how signed/unsigned programs are treated, once they're installed.

John