What's the difference between a battery tender and charger ?

 

A "good" battery tender senses when the battery is below fully charged, I believe 13.8 (could be wrong) and kicks in at a variable and low amp rate to slowly bring the battery up to a full charge and keep it there. A charger you turn on and it throws a predetermined amperage at the battery and continues to do so until it is shut off or the battery explodes.
Important to note that not all battery tenders are very good at what they claim to do.