APRS is an interesting system, a network for wireless transmission of data packets to the Internet. It's global, free, and low latency. APRS is often used to provide location tracking of vehicles but can route any short text message. It's a great example of amateur radio hackers meeting Internet hackers and echoes other much loved networks like Fidonet or Usenet.
APRS lets you send short messages from a handheld radio to the Internet through the simple expedient of broadcast and repeat. A user broadcasts a new APRS message on a VHF band. Nearby fixed stations ("digipeaters") hear the message and rebroadcast it to other nearby digipeaters, so the message floods the digipeater network (within path and hopcount limits). Eventually an APRS iGate hears the message and forwards it into the Internet where it is published to the world as part of a single, global APRS message stream. Not very efficient, but reliable and well suited to short messages.
What is APRS used for? A big application is location tracking, with radios beaconing out location and speed every few seconds. Here's a friend's training flight. Location packets are a big part of the APRS spec, but there's also arbitrary text and email and SMS-like applications. aprs.fi gives a good map-based view into APRS activity, and here's what The Matrix really looks like.
So what's the drawback to APRS? It's amateur radio, which means strictly non-commercial and you need a license to use it (10 hours of study). Private data is not supported. And it's all hackerware, some assembly required, although recent radios like the Kenwood D72 are pretty turn-key.
The real limit to APRS is that it's the sort of gentle hacker network that's maintained by love. I suspect it wouldn't handle the strain of 100x as many users or commercialization. The broadcast design makes for limited throughput and the community process probably wouldn't handle business interests well. Sort of analagous to the pre-1993 Internet. Nothing wrong with that, but I suspect we'll never see APRS-based location tracking for the mass market.
I've never actually used APRS, just read about it. Many thanks to Adam for teaching me a lot of this