A “green roof” has many meanings. In this article, we are discussing the layers of a growing roof top – which is the classic definition of a green roof.
The top layer is obviously the vegetation. You can often choose your own plants, but they have to be a weight your roof can support when they are fully grown. It is also best to go for local plants that you do not have to water. 
Next is the growing medium. This is a type of soil for your plants to grow upon.
Below the growing medium is a drainage area which also allows for aeration, water storage, and a root barrier.
Underneath that is insulation which is a double protection between the plants and the roof itself.
Below the insulation is another membrane which bars the roots from crawling through the rooftop.
Next is your standard roofing membrane.
Finally, your structural support.
This is a general idea of the “green roof”, but there can be variations. Just remember, it is important to make sure the structural support and really support all the weight you are going to be putting on the roof.
If you are looking to make your home green without putting plants on your roof, consider a metal roof.