I like many of the responses and resources that have been given. In trainings and in discussions, I tend to give some simple distinctions that people can remember and take with them:
Burnout is physical and mental exhaustion.
Compassion fatigue is emotional exhaustion - not having the capacity to care for and empathize with someone
Vicarious/Secondary Trauma results in a compromise in one's sense of safety.
Sometimes, long clinically-based definitions - though good for guiding discussion and practice decisions - don't easily create good transfer of learning