Validators are catalog objects, representing boolean functions (they validate whether given conditions apply or not). They are used by other catalog objects, such as abilities (for auto-cast validation), behaviors (to decide whether the behavior works or not) or effects. Each validator entry is one condition, that can be tested by incuding this validator in appropriate field.
Validators inherit the target references form whatever object used them.
Wrapup validators work only with other validators, they are used to construct more complex conditions.
Simplest wrapup validator, it combines multiple other validators with AND/OR logic function. It can also be used to negate other validator.
Condition validator first test one helper validator, and then, regarding the helper's outcome, decide which of the two validators to use. (If-then-else equivalent)
Simillar to condition, but allows for more complex conditions (switch equivalent).
Generic validators don't belong to any other category.
Counts all units in area that fulfill given condition (=validator). Then compares the count to a give value.
Compares the Terrain texture set of the map.
Time of Day
Compares the time of day, propably in-game time.
Most of unit validators are self-explanaory, I would like to point out:
This applies the standard filter (attributes, unit state, invulnerability....)