Click-through rate (CTR) is a metric that measures the number of clicks on an ad divided by the number of times the ad is shown (impressions). It's a way to measure the engagement with an ad and how well it resonates with its target audience. Conversion rate is a metric that measures the number of people who take a desired action (such as filling out a form or making a purchase) after clicking on an ad. It's a way to measure the effectiveness of an ad in driving users to take a specific action. Cost per acquisition (CPA) is a metric that measures the cost of acquiring a new customer or converting a lead. It's calculated by dividing the total cost of an advertising campaign by the number of conversions. CPA is a way to measure the efficiency of an ad campaign in terms of the cost of acquiring a new customer. In summary, CTR measures how well an ad resonates with its target audience, the conversion rate measures how effectively an ad drives users to take a desired action, and CPA measures how efficiently an ad campaign acquires new customers.