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If you enter a single URL as a standalone line Discourse generates a so-called Onebox, which is a rich representation of the link and content. This works with most sites but, in order for it to work well, the site should implement the OpenGraph and/or oEmbed protocol. Most of the major web platorms (like YouTube, Facebook, Wikipedia, etc) do implement one of those protocols.
Here's a what a few Oneboxed links look like :
