What is WordPress post type ‘aside’?

Aside is a short post or a note published for a short and sweet information purpose. The post formats section of the Codex describes an aside as:

Typically styled without a title. Similar to a Facebook note update.

But, in modern terms, it can be used as much as of your heart’s content. Give it a title or just publish a short note, it’s all up to your taste. Making it short, sweet and informative will meet up with its nature.