What kind of battery trickle charger do I need for a 2002 Honda shadow? Photo included.

http://m.imgur.com/YJECq7t,YhaSKG4

My friend bought this bike from her dad. Turns out he every night would put the battery on a trickle charger/tender (not sure if there is a difference). What kind of plug is this/what kind of trickle charger am I looking for exactly? Want her battery to be topped off every night.