Publication: An entity, such as a magazine, book, or website, that publishes written or visual content.
Format: Type of resource (blog post vs. news article vs. scholarly article, etc.).
It is important to think about the publication and format of a resource because it can help determine its validity or credibility. It's also a good way to determine if the source is right for your particular assignment or topic.
Think of it this way, a news article appearing in a reputable newspaper or magazine has to meet a certain level of standards and has multiple people (such as editors of the newspaper and factcheckers) reading and vetting the article before it's published. A blog post is usually one person's point of view. The blogger may provide evidence or cite their sources, but there is no standard, level of expectation, or vetting process behind their blogpost.
You also want to consider when the source was published (or updated). Typically, the more recent a resource is, the more accurate it will be.