Blogs make use of CSS and templates for their presentation while they store their content either in a database or flat files depending on your set software.
A template defines the data structure and uses the CSS file to define the way information looks. This should already be a familiar design paradigm since it is already implemented in most sites designed today. A more complete examination of this topic is available on A List Apart.
Specific to our set up you can see all of Walker Blogs have a similar layout and content structure. In fact they all use one set of template files, it's just the CSS files that are different between them.
We use wordpress but most blog software makes use of templates built from standard components. Most blog templates define a relatively conservative page layout by default. A chronological list of events with a sidebar on the left or the right. There are a standard set of options that you can enable or disable. In our case we reordered things a bit but decided on a pretty standard set of options.