StandardController クラス紹介します。
Visualforceページコントローラ
public class myControllerExtension {
    private final Account acct;
    
    // The extension constructor initializes the private member
    // variable acct by using the getRecord method from the standard
    // controller.
    public myControllerExtension(ApexPages.StandardController stdController) {
        this.acct = (Account)stdController.getRecord();
    }
    public String getGreeting() {
        return 'Hello ' + acct.name + ' (' + acct.id + ')';
    }
}Visualforceページ
<apex:page standardController="Account" extensions="myControllerExtension">
    {!greeting} <p/>
    <apex:form>
        <apex:inputField value="{!account.name}"/> <p/>
        <apex:commandButton value="Save" action="{!save}"/>
    </apex:form>
</apex:page>