Outreach and guest posting are both strategies used in link-building campaigns. There is a cross-over as outreach link-building may entail publishing a guest post on third-party websites.
That said, there is still a difference because outreach entails proactively sending a message to someone in charge of a website requesting them to link back to your content. The goal is to obtain a link.
With guest posting, the intent is mostly to provide valuable content. There is often no need to reach out as many websites provide a “Submit an article” form. When submitting a guest post, the content may or may not include a link.
High-domain magazines may discourage backlinking attempts. Still, the content is identified as a guest submission. There will be an author’s bio section where you can write a short description. It may or may not have a URL to your website depending on the website’s policy.