When the admin changes(Add/Remove) roles of the user, how to get assigned and removed roles of that user? There are more than 20 user roles(Editor,approver,new admin..) in the  project. Whenever the admin assign/delete the role to user then the user should be notified with email. Email should say Hey Prabhakar You have been assinged to following roles or you have been removed from following roles.
Implement hook_user_presave:



/**
 * Implements hook_user_presave()
 */
function yourModuleName_user_presave(&$edit, $account, $category){
  foreach ($account->roles as $role_id => $val) {
    $user_roles_existing[] = $role_id;
  }    
  foreach ($edit['roles'] as $role_id => $val) {
    // $edit['roles'] contains all roles ids as key and selected role id as val. If not selected then value is 0
    // ex. $edit['roles'] = array(5=>5, 6=>0) here role id 5 selected
    if ((int) $val > 0) {
      $user_roles_form[] = $role_id;
    }
  }    
  $user_roles_new = array_diff($user_roles_form, $user_roles_existing);
  $user_roles_removed = array_diff($user_roles_existing, $user_roles_form);     
  //roles added
  if (count($user_roles_new) >= 1) {
    /**
     * Required code...
     */
    drupal_set_message('Testing new roles count' . count($user_roles_new));
  }
  //roles removed
  if (count($user_roles_removed) >= 1) {
    /**
     * Required code...
     */
    drupal_set_message('Testing Removed roles count' . count($user_roles_removed));
  }
}