Definitions from the desk of Talapas:
Bus: Unit designed to get Characters into combat. Classic bus is Bret Knight unit with lord, sorceress, and double hero.
Screening unit: Unit designed to cover, draw fire, and redirect. Also used to protect main units from being flanked. Composition depends on army and purpose. Skink skirmishers are an example of a screening unit.
Deathstar: A very large unit that is the ultimate in points denial and destruction. It may or may not include characters. Usually is 40+ of elite infantry troops, or 15+ or monstrous troops. Sometimes appears as a horde.