Backend versus frontend modules
There are two types of modules that you can create: backend modules and frontend modules . As the name describes, backend modules are only shown in the backend administration section of your website which an ordinary user does not see. Frontend modules are displayed on the website, however you can adjust permissions so that only certain users will see the module. You can't have one module that is shown on both the backend and the frontend; you would need to create two separate modules to achieve this although the code would be almost identical with just a couple of minor changes.
Backend modules' installation location
Backend modules are installed in the /administrator/modules folder of your site.
Frontend modules' installation location
Frontend modules are installed in the /modules folder of your site.
