The Common panel of the Render Setup dialog contains controls that apply to any rendering, regardless of which renderer you have chosen, and that lets you choose renderers.
The Common Parameters rollout sets parameters common to all renderers.
This dialog lets you change the preset resolution on a button in the Output Size group of the Common Parameters rollout.
This rollout lets a rendering job send email notifications, as network rendering does. Such notifications can be useful when you launch a lengthy render, such as an animation, and don't care to spend all your time near the system doing the rendering.
The Scripts rollout lets you specify scripts to run before and after rendering. Each script executes once at the beginning or the end of the entire rendering job for the current scene; the scripts do not run on a per-frame basis.
The Assign Renderer rollout displays which renderers are assigned to the production and ActiveShade categories, as well as the sample slots in the Material Editor.