$block->delta: An ID for the block, unique within each module.
$block->region: The block region embedding the current block.
$classes: String of classes that can be used to style contextually through
CSS. It can be manipulated through the variable $classes_array from
preprocess functions. The default values can be one or more of the
block: The current template type, i.e., "theming hook".
block-[module]: The module generating the block. For example, the user
module is responsible for handling the default user navigation block. In
that case the class would be 'block-user'.
$title_prefix (array): An array containing additional output populated by
modules, intended to be displayed in front of the main title tag that
appears in the template.
$title_suffix (array): An array containing additional output populated by
modules, intended to be displayed after the main title tag that appears in
$classes_array: Array of html class attribute values. It is flattened
into a string within the variable $classes.
$block_zebra: Outputs 'odd' and 'even' dependent on each block region.
$zebra: Same output as $block_zebra but independent of any block region.
$block_id: Counter dependent on each block region.
$id: Same output as $block_id but independent of any block region.
$is_front: Flags true when presented in the front page.
$logged_in: Flags true when the current user is a logged-in member.
$is_admin: Flags true when the current user is an administrator.
$block_html_id: A valid HTML ID and guaranteed unique.