APRS, or Automatic Position Reporting System gives an amature a way of making available to other amatures, and anyone interested, a way of figuring out about where you are at any given time.
As a brief overview, if you take a Global Positioning System (GPS) receiver that outputs a standard called NEMA, and pump that into a small computer with one or another form of a Terminal Network Controler (TNC) connected to a radio that at the very least will transmit on the frequency 144.39 as a packet network transmition. There are both packet repeaters, and Internet gateways for this traffic that amoung other systems transfers the location information to FindU.com.
If you install such a setup in your car, you can track approximately where your car is, or has been. Sometimes this leads to some odd results. As an example today I drove into work. One of the packets that reported my various locations through that trip has me located in the middle of Medicine Lake. This may not be too unusual considering it is March, and people have been known to do this thing called Ice Fishing on the lake. However the reality is that GPS location information is not always correct.
I have seen reported maximum speeds on my GPS receiver in excess of 900 mph. Which is a bit slow for a Concord, but the Chevy S-10 we were in really wasn't capable of that. Well, possibly as it's terminal velocity in a free fall, but I don't think the pickup left the pavement on that trip. I have also seen tracks where a stray reading indicated a location several blocks off of the remainder of the track. Of course the next reading was back along the track I remember driving, but the system is really not perfect.
So if someone asks 'What were you doing at Cub?' when you were at Rainbow, explain that you like to walk through the store and stand in the long checkout line to pick up a pack of gum, and annoy the other patrons. And don't trust that APRS will give you 100% accurate information on where your spouse drove the car last night.
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-Rusty - kc0vcu