Oh, look - the ACLU just updated their page on these things.

Stingrays, also known as "cell site simulators" or "IMSI catchers," are invasive cell phone surveillance devices that mimic cell phone towers and send out signals to trick cell phones in the area into transmitting their locations and identifying information. When used to track a suspect's cell phone, they also gather information about the phones of countless bystanders who happen to be nearby.
https://www.aclu.org/issues/privacy-...-whos-got-them