Selenium has driver.getWindowHandles() and driver.switchTo().window("{windowHandleName}") commands to work with multiple windows.
The getWindowHandles() command returns a list of ids corresponding to each window and on passing a particular window handle to driver.switchTo().window("{windowHandleName}") command we can switch control/focus to that particular window.
for (String windowHandle : driver.getWindowHandles()) {
driver.switchTo().window(handle);
}