1. Classes
(About Game of Thrones.NET/Game Rules)

... They keep the Watch’s history, maintain the domestic arts, and in some cases, squire or apprentice to the high officers. Red Priest of R'hllor Clergy of the R'hllor religion are called 'red priests,' ...

2. R'hllor
(Faiths of Westeros/Faiths of Westeros)

... of stone. Clergy of the R'hllor religion are called 'red priests,' due to the loose, crimson robes they wear. In the east, children are often given to temples of R'hllor to be raised into the priesthood. ...