Hey Jennifer, difference between getWindowHandle() and getWindowHandles() methods is as follows:
driver.getWindowHandle() – It returns a handle of the current page (a unique identifier). It is used to get the address of the current browser where the control is and return type is String. Syntax is:
String handle = driver.getWindowHandle()
driver.getWindowHandles() – It returns a set of handles of the all the pages available. It is used to get the address of all the open browser and its return type is Iterator<String>. Syntax is:
<List> handles=driver.getWindowHandles();