Shortcut to switch windows, bringing the newly active window to the top
⌘ + TAB switches between applications. It brings up last active window of selected application. 

Application can have many windows open (or no open window at all). To navigate between application windows: 


first navigate to desired application using ⌘ + TAB  
without releasing ⌘ use Up or Down arrow on keyboard to see all open windows of selected application. 
Use Up/Down/Left/Right arrows to navigate between windows. 
reaching desired window press Enter to bring it in front