Green Tea Leaves
All types of tea come from the same plant, Camellia Sinensis, and what determines the different types of tea is by what happens to the leaf after it is picked.
When the leaves are brought in from the gardens, they are withered or partially dried by using hot or cold air for varying amounts of time depending on the climate, humidity and the desired end product.
The withering prepares the leaf for the rolling stage where the leaf is crunched in various fashions depending on the grade of the tea. If the leaf wasn't withered (partially dried) before the rolling phase, the leaf would just turn into green mushy soup when rolled.
The rolling phase releases the flavors from the leaf in order to then capture/retain the flavor in the final step, the firing. In the case of Green tea, the rolled leaf goes directly to the firing stage so the leaf stays green.