From 1d4chan

Priests are authority figures in religious groups and movements; these are the individuals who (claim to) have a special connection with whatever gods their faith holds dear, and thus act as their mouthpieces in the worlds of men. Common duties include spreading the word to convert others to their faith, diddling altar boys and ministering to the physical and spiritual needs of their followers.

In Advanced Dungeons & Dragons, "Priest" is used in a similar fashion as Mage or Warrior; it is a catch-all term for all classes that utilize divine magic, serving as a shorthand for Cleric, Druid, and Specialty versions thereof.