The Dragon Priests were originally groups from 3000 years ago who each had their own order for the 5 different colours of balance. They maintained temples which held all sorts of ancient knowledge and artifacts, as well as working with their according Booster to help end the Dragon-Human War.
Today, there are still Dragon Priests around but they remain hidden for safety reasons, as they generally still guard ancient knowledge along with ancient artifacts. Despite each type of Priest following a particular order according to their draconium colours, even the same draconium colour can have multiple temples around the city and the wastelands, as seen by the Dragon Booster visiting a hidden gold draconium priest temple whilst Mortis holds his own gold draconium temple hidden under Penn Stables.
Gold Draconium Priests are the ones most commonly seen throughout the series, leading to the misconception that there is only one type of Dragon Priest. However, in Episode 36: Professor Stubborn, a Green Draconium Priest is seen speaking to the protagonists. No other colour priests are ever shown during the series. The temple forms and presentation varies from temple to temple, between and within draconium colours.
The current leader of the Gold Draconium Priests is Tannis Blayrr, one of Connor's closest allies within the Ancient Order of Priests and also the Priest who speaks to the Dragon Booster in place of the other Priests.