A Cleric is a priest who serves a god or goddess, and casts spells by that deity's power. Clerics - along with shamans and druids - are the primary healers of the GA.
A Cleric is the conduit used by a divine entity to enact its will upon the world. Whether that is to protect or destroy depends entirely upon the deity; in either case the Cleric is the Hand of Divinity.
They have many tools at their disposal. They can wear up to chain in armor, granting them D1 on the meter, but may use only a single weapon type as befitting their deity, plus an array of helpful and harmful spells. Magic Resistance of 2 is allowed.
A cleric can use the single chosen weapon, plus the HUD and a single limited-power artifact such as a wand or book. If the chosen weapon is two-handed, then no other hand-held item can be used.
They cast from divine magic and can heal themselves and those around them. They can grant their companions endurance and food. Not all spells are for healing, however. A priest can call down the wrath of their deity upon an enemy as well.
No spells are allowed to be cast from the Air unless that cleric is engaged in Air to Air combat using the Flying Caster meter setting.
Check with your sim rules and the guild you are looking to join for individual rules for ranking and a full list of weapons and races allowed to this class.