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What is the difference between Terror & Fear?
Detailed explanations of Terror and Fear are on page 50 & 51 of the rulebook.
To sum it up, it gives you possible bonuses when charging and being charged. Terror is a amplified form of fear (Terror causing units cause fear in fear causing units) and so on.
The rulebook explains it quite nicely.
Hope it helped!
Your fluffraping hurts my eyes. - TehDarkPredator
BRB page 50-51. Terror is in addition to Fear, but an affected unit only tests once per battle. Terror also generates a psychology test when a terror-causing creature is within 6", in addition to charging or being charged.