What is a Post Format?
Post formats are features that some themes offer. Themes that support post formats enable you to customize their style depending on your chosen format.
More About Post Formats
Post formats were introduced to WordPress in version 3.1. Formats are meta information that themes can use to determine which type of post you’re working on.
WordPress provides a master list of post formats that themes can choose to adopt or not. A theme might include only a handful of formats, and users will be able to choose between them when creating or editing posts.
Some of the most popular post formats in WordPress include galleries, statuses, quotes, links, audio files, and audio transcripts. For a theme to add support for formats, it must follow a specific set of development guidelines.