In the episode of Stargate SG-1 called "The Fifth Race", there are 4 advanced races which are listed, namely the Asgard, the Ancients, the Nox, and the Furlings. The Asgard and the Ancients were extensively mentioned, the Nox had a few episodes that they were mentioned, but so far as I know, the Furlings never appeared, nor were heard from. Who are they?
The Furlings never did make an appearance on SG-1, but there was mention of them in several episodes, and Furling ruins were found and explored.
The length of time that the Furling nature has remained a mystery in the series has given the producers the opportunity to tease fans with a running gag. When Executive Producer Robert C. Cooper was asked "Will we ever meet the Furlings?", his answer was "Who says we haven't?"
There is also a lot of information on the sg1 wiki.