The conversion rate is the percentage of users who take the desired action.
The desired action can take many forms, varying from site to site. Examples include sales of products, membership registrations, newsletter subscriptions, software downloads, or just about any activity beyond simple page browsing.
Example: An ecommerce site is visited by 100,000 people during the month of April. During that month, 2,000 users purchased something from the site. Thus, the site's conversion rate is 2,000/100,000 = 2%.