Battery Tender is a Brand name
They work great, 2 Kinds of Battery chargers, The kind you mount in your boat, like The the ProMariner ProSport 20 talked about above, and the Battery tender you talked about that is made to sit in and around and then you unplug it and go on your way. They are cheaper and work well. Depending on what kind of batterys you have, Example im not sure the Battery tender is a SMART charger that has difrent program cycles for diffrent types of batterys.
If you have a traditional Wet/cell battery a battery tender will do the trick. If you have AGM batterys like a Optima I would go for a more expencieve smart charger