Leafé is the source of all life. The Prétear is a young girl who is born with the power to create and manipulate Leafé, either by 'preting' with one of The Leafé Knights, or by becoming the 'White Prétear' by herself. It is not known how many Prétear's were able to become the White Prétear.
The fundamentals of Leafé is that it exists within all living things, even where you wouldn't expect it to (the elements that each of the Leafé knights' control all have their own Leafé). When a living thing is drained of all of its Leafé, it will die.