They're really cool little gadgets for sure. I use one with my motorcycle in the winter. They do just what they say, they charge your battery if it's low and then they monitor the voltage in your battery and give it a little trickle of juice when it needs a boost. There's a lead that you attach to the battery and tuck out of the way that plugs into the unit when you're parked. I have mine running through the frame of my bike under the seat pan and it pops out right in front of the rear fender. The plug is about the size of two Sharpie caps. They work great!
http://www.batterytender.com/