Who are the people of the Ress?

I am wondering who the people of the Ress that the Qur'an mentions are? unlike other peoples/tribes it does not say what Prophet is sent to them, or who they are. Who are the People of the Ress? and what Prophet was sent to them? > كَذَّبَتۡ قَبۡلَهُمۡ قَوۡمُ نُوحٍ۬ وَأَصۡحَـٰبُ ٱلرَّسِّ وَثَمُودُ > (١٢) > > Before them was denied (the Hereafter) by the people of Noah, the > Companions of the Rass, the Thamud (12)