What are advanced validation rule functions: ISCHANGED, ISNEW, PRIORVALUE, ISPICKVAL?

ISNEW(): Returns true only when a record is being created (not on updates). Used to enforce required fields only on creation. ISCHANGED(field): Returns true if the field value changed from its previous value. Useful for preventing edits to locked fields after a certain stage. PRIORVALUE(field): Returns the field's value before the current edit. Used to compare before/after values in rules. ISPICKVAL(field, "value"): Returns true if a picklist field equals the specified value. Combined example: AND(ISCHANGED(StageName__c), ISPICKVAL(PRIORVALUE(StageName), "Closed Won")) — prevents re-opening a closed opportunity. These functions are only valid in validation rules and formula fields, not in custom formula fields that reference them outside that context.