PHP "Smart Refresh" for looping server lookups

This may sound a bit long and drawn out, but I'll try my best to summarize what I'm having trouble with. Roughly 2 weeks ago I began writing scripts to run a free TagBoard service. (Examples: Tag-Board.com and [mine] MyTagBoard.com) - my 2-week-long goal was to compile a list of features (based on 3 or 4 tagboard sites) and add those to this set of scripts. I was successful in pretty much all the features and more, except for one: "Smart Refresh". The tagboards currently refresh at a rate of xx seconds, set by the tagboard owner. Other sites have managed to design the tagboard to refresh ONLY when a new tag exists on the server, simulating a "live chat". My question is, how is this "Smart Refresh" done? How does a page that's loaded, just "sit there" and still be able to take a peek at the server without just refreshing the whole page? It's mind-boggling, even for me. :-D I'm not looking for actual code; that part I can do; I'm just looking for a basic "what does what" procedure on how it's done. Any thoughts on this would be very greatful! Thanks in advance =) ~ Tim