You can get a list of bugs depending on the tags that those bugs have or don't have.
Next to the Tags search box are two radio buttons: Any and All. Select Any and Launchpad will return bugs that match any part of your search term. Select All for bugs that match your entire search query.
To search for the absence of a particular bug tag, place a minus sign
before its name: e.g.
test uiand select the All radio button.
test -uiand select the Any radio button.
You can mix the different types of search.