Theming Drupal 7: Block vs Node vs Field et al

What is the difference among pages, regions, nodes, blocks, and fields?

There's one page that may contain multiple regions.  Regions are containers for nodes and/or blocks.  Multiple nodes can appear within a region if the URL being accessed is a listing view, in which case the nodes will be rendered in teaser form with "Read More" links leading to the full view.  Multiple blocks can also appear in a region.

Blocks seem to correspond to 'widgets' in other systems: chunks of content that can be dropped into the sidebar and remain fairly static on many pages.  For instance, Drupal's search box is a block, and it lands in the sidebar_first region of both default node types.  Blocks do not have fields.

Nodes, on the other hand, correspond to the content that actually gets added to the CMS.  Blog posts, static pages, whatever.  Nodes get URLs assigned to them, either through the URL aliasing features, or the default node/4 style path.  (Or through any SEO friendly URL generation modules you may have installed.)  Nodes have a type and node types have fields.  Fields receive values per-node that are displayed on the node type.

Everything related to a node is rendered inside a single container, corresponding to the page content variable.  Overall, the URL, node with its type and fields, and page content variable are all one interrelated thing when viewing a single node.  Pages and regions are related to the theme as a whole.  Blocks are strongly related to a theme, but also customizable based on node types through the block's Configure link.

Blocks, nodes, and fields are fairly customizable and appear in the admin interface under the "Structure" menu item.  Controlling blocks is done with the Blocks item in the menu (straightforward enough); controlling nodes is done through Content Types.

I should probably note that the paths through the admin interface given are for the default setup where Bartik is the main theme and Seven is the administration overlay theme.  Those paths are not guaranteed for other themes, since themes are PHP and can do nearly anything they want.

If you want "a content area" with several "pieces" to it, the path of least resistance is to construct node.tpl.php with the contents of that content area inside, using fields to display each individual "piece" desired.  Then in your admin interface, establish the fields so they show up when editing the page.

To make that clearer, let's say MyCorp wants a video on their front page with a blurb to the side, a graphical separator, and another couple paragraphs below.  I could make a mycorp_video content type, and add two fields (field_video_embed and field_video_blurb), then create a node--mycorp-video.tpl.php file with the container divs, central bar, and calls to <?php render($content['field_video_embed']); ?> inside their respective containers.  Then I could leave the couple of paragraphs as "body" content and print that below the separator.  Once the template is ready, the node type can be created in the admin interface, the node actually added, and finally set to be the front page of the site.

Controlling something outside of the content area based on the content (node) type is not possible by default, but can be done with an override in the template.php file for the theme:

function themeName_preprocess_page(&$vars, $hook) {
  if (isset($vars['node'])) {
    /* If the node type is "blog_madness" the template suggestion will be "page--blog-madness.tpl.php". */
    $vars['theme_hook_suggestions'][] = 'page__'. $vars['node']->type;
  }
}

The above code was posted by JamieR at drupal.org/node/1089656.  By default, drupal's page renderer only knows about specific nodes (like page--node--4.tpl.php) and not content types (aka node types) in general, which is what this override adds.

A second approach is to use the CCK Blocks module to convert fields into blocks.  This allows them to appear on the block layout and be placed in regions in spite of being node-specific.  The blocks are then made visible in region templates with a  cck_blocks_field prefix, for instance cck_blocks_field_video_embed for a video_embed field.

The latter approach is actually the one I ended up taking.  I needed to handle several optional areas in various combinations.  Instead of a big list of node types and duplication of the markup for any fields shared between types, I have two basic node types and regions handle sharing the markup and displaying the available fields (or nothing, when no fields are set.)