The percentage of customers who actually purchase, out of all of those respond to an ad or click on a link. 
Example: 1,892 prospects respond to a direct mail piece or click on an ad. Of those 1,892 prospects, just 71 of them actually purchase the product. The conversion rate is 71 divided by 1,892 or 3.8%. Or put another way, 3.8% converted.

When the act of clicking or responding results in a sale without an interim step, then the conversion rate is calculated by dividing the number of prospects who purchase by the universe of coupons, ads, etc., that were distributed.