This is a GPS device that can send SOS signals in an emergency. You can send pre-defined messages (cellphone text messages and email) to specific people, saying things such as "I'm ok, don't worry", or "You need to come help me out," and such. The messages include your GPS coordinates so your friends can find you. In addition, there is an SOS button, which sends a distress signal to the authorities along with your coordinates, and they will send out whatever is needed to rescue you (helicopters, tanks, elephants, etc.).
You can also use the device to send progress of your travels to folks. The device sends your coordinates every ten minutes or so to a website, where they are overlaid on maps, for your friends to watch.
I bought this device because I do a lot of hiking alone, but it's also a good investment for jeeping. I'm getting a newer unit, though, not because there is anything wrong with this one, but because the DeLorme company just came out with their own, and it has a couple of different features that I like, plus they are a Maine company, and I like helping out fellow Mainers.
Below is the SPOT website. the units that they show are also a bit newer than the one I have, which I bought in 2008. Mine is a standalone unit, and can't be combined with a cellphone, or send on-the-fly messages.
There is a yearly fee beyond the cost of the unit, but, it is an excellent bit of insurance for anyone who travels the back country. If anyone's interested, I'll take the best offer.
http://www.findmespot.com/en/index.php?cid=102