Whenever an application is developed by using Webcenter , admin of the particular web application will have rights to perform some changes to the application which are visible to all the users across the world. Mostly the changes are related to the changing templates, colors, layouts, images, tetc.....
Thus Customization means user with admin privileges who can perform changes to the application which are visible globally.
Coming to the personalization , every registered user to the webcenter application will have some privileges to change the things which are visible only to that user after login every time.
A good example for this topics is igoogle where users can personalize the blog by arranging the applications like calendar, weather apps, News, etc...in a particular order as he wish. These personalization whatever done by the user should be saved and generated whenever he login the next day. This can be achieved by using MDS repository directory.
Customizations can visible globally which are done by the admin are achieved by a webcenter component called Oracle Composer.