There are two types of modules, System Modules and Regular Modules. The System Modules are modules that you can move around a page, but not remove from a page. Some of them are customizable, while others are not. Regular Modules are modules that you can add to and remove a page. You can also change their location on the page and customize most of them.
The following is a list of modules available in vBulletin 5. Each article includes information of what you can customize.