Its different from jurisdiction to jurisdiction but basically running an unlicensed taxi or livery service.

Another issue that seems to be a major hangup in my jurisdiction (which has banned Uber from operating) is the lack of "proper insurance" for Uber drivers/cars.

Lyft's entire business model skirted regulation. At least when Uber started they required all drivers to have passenger carrier permits and commercial insurance. Then Lyft comes around and completely ignores the law. Completely. Uber starts losing business, and so then Uber responds in kind with UberX.

Now it's legal because it flies under the banner of "rider sharing". But we all know that there is really no sharing to be had. It's just a for hire service. The regulation will return, eventually.