cat_row
cat_row is a filter that filters each row of the categories list under Manage -> Categories. More usefully, it also filters rows which are added to the list via AJAX when a new category is added from Manage -> Categories -> Add Category.
Context:
File: wp-admin/includes/template.php$output = "<tr id='cat-$category->term_id'$class>
<th scope='row' style='text-align: center'>$category->term_id</th>
<td>" . ( $name_override ? $name_override : $pad . ' ' . $category->name ) . "</td>
<td>$category->description</td>
<td align='center'>$posts_count</td>
<td>$edit</td>\n\t</tr>\n";
return apply_filters('cat_row', $output);
}
This hook is a filter which means that information is passed through it, and then used by WordPress. Your function needs to accept that information, and return it. Using add_filter('cat_row', 'your_function'); helps you to remember this distinction. When you are passing an ID, it is assumed that you will return the ID as it was given to you. With filters that pass strings or arrays, you may manipulate the information before passing it along.