Jiu Jitsu is a Japanese martial art that was developed by the Samurai to allow them to fight effectively when they were disarmed on the battlefield or had no access to their weapons.  Based on locking throwing and striking techniques Jiu Jitsu provides a wide range of options for a practitioner to use against their attacker.  It is based on the principle of using an opponent's attack against them, the literal translation of Jiu Jitsu approximating to "Yielding Art".

Defendo is a fighting system created by Bill Underwood.  It was derived from another system called Combato that Bill developed in the early 1900's and used extensively in WWI (as well as training special forces troops to use it during WWII).  Whereas Combato was developed for use on the battlefield by special forces troops, Defendo was developed post war for use by law enforcement and citizens.  It is effectively a watered down version of Combato with techniques removed or modified so as to no longer purposefully cause permanent injury or death.