This is Aside post format. The only difference between post formats in this theme is how they are displayed on front page and on category/archive page.
Post Format is a theme feature introduced with Version 3.1. A Post Format is a piece of meta information that can be used by a theme to customize its presentation of a post. The Post Formats feature provides a standardized list of formats that are available to all themes that support the feature. Themes are not required to support every format on the list. New formats cannot be introduced by themes or even plugins. The standardization of this list provides both compatibility between numerous themes and an avenue for external blogging tools to access this feature in a consistent fashion.
In short, with a theme that supports Post Formats, a blogger can change how each post looks by choosing a Post Format from a radio-button list.
Using Asides as an example, in the past, a category called Asides was created, and posts were assigned to that category, and then displayed differently based on styling rules from post_class() or from in_category('asides'). With Post Formats, the new approach allows a theme to add support for a Post Format (e.g. add_theme_support('post-formats', array('aside'))), and then the post format can be selected in the Publish meta box when saving the post. A function call of get_post_format($post->ID) can be used to determine the format, and post_class() will also create the “format-asides” class, for pure-css styling.