If you create a new entity, you may have stumbled across the Attribute System, for example if you want a custom max. health value.
Now a detailed explanation on what the Attribute System actually is can be found here: http://minecraft.gamepedia.com/Attribute
Here I will show you how to create your own Attributes for your entity and modify all of them in code.
NOTE: all attribute classes mentioned in this tutorial can be found in the package net.minecraft.entity.ai.at