The Render Output window shows the progress of the rendering based on the Render settings selected. This window also provides options for applying image adjustments while the image is being rendered that can be saved out when the render is complete.
Close
While the rendering is still in progress you can abort by clicking the red 
this will give you the option to save the rendering at the current state or discard the image. When the rendering is finished the image is automatically saved in the selected location. Click the green 
to close the render output window.
Pause/resume Rendering
Click to temporarily stop the rendering. Resume by clicking .
Save image to file
Select the Save icon to save the current render as it appears in the Render Output window.
Hide/Show Image Styles Panel
Toggles the Image Styles panel. Here existing Image Style may be adjusted or new Image Styles created. Refer to Image Styles for more on how to create and apply image adjustments.
Zoom to fit
Fits the Image to the size of the view-port.
Zoom to 100%
Zooms the image to its full size regardless of the size of the view-port.
Zoom slider
When the view-port is smaller than the full size of the image, the slider allows incremental zooming the image between fitting the view-port and full size.