Spirit families are a collection of one type of spirit who have similar features. Each family is usually composed of one spirit from each element; fire, ice, water, shadow, light and fungal.
There 16 different families with more than 90 different spirits to battle and capture! If you defeat or capture a spirit your element is weak to you will be rewarded with more XP than if you defeat or capture one you are strong against.
NB: There are also normal, silver and gold spirits too. Silver ones are harder than normal and Gold are harder still!