A target audience is a category: "our employees," "our customers." A persona is a specific, human profile within that category. The target audience tells you who you're reaching. The persona tells you how you're reaching them.
"Middle managers in the financial sector" doesn't provide much guidance. "Karin, 42, a team leader, would like to network but has little patience for long plenary sessions" offers much more direction when designing the invitation and program.