Anyone can post a Show HN, which is for sharing something you've been working on. The rules for those posts are here: https://news.ycombinator.com/showhn.html
Launch HN is a one-time thing for YC startups. We work with them to edit their text, in the hope of presenting things the way that HN prefers—for example we coach them to take out sales and marketing language. You can see the previous submissions here: https://news.ycombinator.com/launches.
These posts are treated like YC-startup job ads in that they get an automatic placement on HN's front page. Other than that, they are like regular submissions in that users can vote and comment on them, and they rise or fall on the page like the way that submissions do.
This is one of three formal ways that HN gives back to YC in exchange for funding it. The other two are job ads and displaying YC alumni usernames in orange to other alumni. I need to add this to https://news.ycombinator.com/newsfaq.html.
Incidentally, we're happy to help non-YC startups with their launches or Show HNs too. The only bottleneck is time; my time lemons are starting to be squeezed dry. But we can at least send the same list of tips we give to YC startups, and sometimes more direct feedback as well. Email a draft to hn@ycombinator.com.
Launch HN: YC portfolio companies showing HN what they've built, often within their batch (but can sometimes be years after they went through YC)
https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...
It looks like Launch HN has been a thing for at least three years: https://hn.algolia.com/?dateRange=all&page=0&prefix=true&que...