Displaying different in-page content to cliente/admin

You are going to want to look at a few things. First, the WP_user object which will enable you to check if the user is even logged in. But you can also check capabilities with the has_cap method.

You can even define your own capabilities and apply them to users.

Then you would do something along these lines:

 <?php

 $user = wp_get_current_user();
 if($user->exists()){
      // logged in user
     if( $user->has_cap('edit_users') ){
          // user can edit users
     }else{
          // this user cannot do that
     }
 }else{
     // anon user
 }