Damages seem to be quite straightforward. If Percy was in a car accident and totaled his car, the cost of his car is part of his damages. If Percy were injured, those are damages too.
The problem is, what are damages meant for? In the law, damages are meant to make a person whole, and bring them back to the state they were in before an accident. In the case of things that aren't tangible (injuries, for example), measuring them is more an art than a science. That's where creative lawyering gets involved- how do you show that the damages somebody suffered are as great (or as little) as possible?