It's basically a "personal journal entry" type post.

Here on Steemit, it was started by @surpassinggoogle, I think originally as a way to get people who had little to no experience writing a blog an easy entry point: Just write something about your life and your day, and forget about having a "topic."

Then it has kind of morphed and developed into something more, since a lot of people who have been here a while have picked it up... based on the idea that the web has typically been more about "information" than "people." So Ulogs are now also part of an initiative to build "the Internet of PEOPLE" rather than just "the Internet of INFORMATION."

You could argue that Farcebook and Instagram do that... but they are not really places to write and share thoughts; they are more just "post and run" venues.