I'd suggest you to go through the following docs avalable online: https://developer.salesforce.com/page/Salesforce_APIs
Visualforce is a framework which is used to design the UI side in Salesforce and am not sure what exactly you mean when you say "which api used in visualforce pages" Could you please elaborate on that..
https://developer.salesforce.com/page/Salesforce_APIs
Visualforce is a framework which is used to design the UI side in Salesforce and am not sure what exactly you mean when you say "which api used in visualforce pages" Could you please elaborate on that..