Yes Well Defined , Conversion Rate For Website is percentage of visitors your advertisement sells, compared to the total number of visitors on it. A conversion rate is measured by the number of potential visitors performing the desired action, whether the action is buying a product, buying something , signing up for your list , calling your office , downloading a PDF , clicking through to another page etc .
In Terms Of E-commerce We Can Say CR = Number of Orders Divides by Number of Visits or say (Number OF Visitors turns to Customers from website ) .
Thank For Link , Yes Agree Website usability , landing pages are more Important for better Sales and Conversions .
Also Build the pages with Directed goal As conversion happens when the visitor reaches the goal. Also Analyse the traffic of your website with helps of tools like "Web Analytic Tool " to knowing the conversion rate whether happening of your Referral traffic , brand traffic , PPC traffic , organic traffic , ads and put more emphasis with those from where get more conversions .