Hi,

       ApexPages.currentPage().getParameters().get('id')

The getAccount method uses an embedded SOQL query to return the account specified by the id parameter in the URL of the page. To access id, the getAccount method uses the ApexPages namespace:

• First the currentPage method returns the PageReference instance for the current page. PageReference returns a reference to a Visualforce page, including its query string parameters.
• Using the page reference, use the getParameters method to return a map of the specified query string parameter names and values.
• Then a call to the get method specifying id returns the value of the id parameter itself.
  ApexPages here means it refers for the current page with the currentPage ApexPages method.
  
  Apexpages.standardcontroller:
  
• The only time it is necessary to refer to a StandardController object is when defining an extension for a standard controller. Here ApexPages means A controller extension is any Apex class containing a constructor that takes a single argument of type ApexPages.StandardController.  The extension is associated with the page using the extensions attribute of the <apex:page> component.
  

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