Rendered images appear either in a viewport or get displayed in the Render Window.
When you render a scene, the image can be displayed in either the viewport or the Render Window. This is the render destination.
The default setting is Window. When the render destination is set to Window, the renderer automatically opens the Render Window and the image is processed. Upon completion, the image is displayed. Images that you want to keep can be saved from the Render Window.
If you choose to set the render destination to Viewport, the generated image is rendered and displayed directly in the active viewport. Once rendering is complete, you can save the image or save a copy of the image to one of the following file formats: BMP, TGA, TIF, PCX, JPG, or PNG.