Mathematica is divided into two pieces: the kernel which performs all the calculations and the front-end with which the user interacts. The front-end is responsible for displaying and styling the notebook interface, and launching the kernel when a calculation needs to be performed.
Since the front-end is a vast system, please consider using some more specific tags too. Use this tag (with the appropriate specific tag) for questions on customizing the front-end options or modifying
*.tr files (customization), creating keyboard shortcuts (keyboard), creating stylesheets (stylesheet), syntax coloring (syntax), etc. Questions relating to nootebooks, their structure and behaviour, dialog and normal notebook windows should use the tag notebook. Questions relating to palettes should use the tag palettes.