A plugin can be called a module in other very similar applications, but the definition remains the same. One of its main purposes, is to allow the user to perform actions in a different way than those provided by WordPress, or to implement new operations.
An example of the first type is to add user roles to WordPress in addition to those already customary; in this case, therefore, is improved a feature already present. Similarly, the slideshows that are managed by a particular plugin, belong to the second type, since WordPress does not allow this operation is then implemented.