I'm researching the idea to add tags and filtering by tags to some lists on our apps and I can't find a good answer to either use AND or OR when multiple tags are selected.
To say it differently, when a user selects two tags in the search form, do we show them entries having both tags, or entries having at least one of them?
Have you already implemented such a thing? Do you have good examples of success with it?
IIRC most sites treated two separate tags as a loose AND. i.e. items tagged both 'TAG A' and 'TAG B' would sort to the top (whatever the display method was) while some sites would throw in results that had one or the other tag _after_ the results showing both tags. I think flickr still has the original tag search that they developed when tags were a novel thing. Technorati had a nice tagged based search, you might search for anything written by their staff before the site imploded.
If you have the opportunity to consult with anyone with taxonomy or information architecture experience, do so.