It's a business that transfers wealth, at great speed, many many times per second, 24/7. HN users have a massive hard-on for them because their engineers are so skilled, but the broader social utility of their business is basically nil.

They trade stocks quickly with computers.

AFAIK they’re like a hedge fund with quants that trades its own money. In fact, most people (everyone?) who works there is a quant. It’s the only way they trade. They are extremely profitable. Of course you can Google this and get a more accurate picture. I know them as the most famous OCaml shop.

Personally I have mixed feelings about what they do. The engineer in me used to root for them. The way they operate as a pure tech shop was very refreshing in the hedge fund/traditional finance world (crypto world is the opposite). But the trader in me now abhors how they make their money, that is arguably at the expense of retail traders.

What's the grumpy version of xkcd 1053? Anyway, Jane Street does high frequency trading on Wall Street, and they're kind of a big deal in that sector. They date back to 1999, with some IBM people. Other than the money, the reason they're interesting for HN is they're an OCaml shop and do a lot for that ecosystem. So if you're a computer language nerd and want to work in a functional programming language, but one that has real world applications, and make a lot of money doing it, Jane Street is the place to be!