.PSD is for whenever you need to save something and need to go back and add changes and such without having the complete product. 

.PNG is the complete product of the thing you have created and it is the best quality for images. 

A lot of people have asked for this and since now that I'm doing tutorials and resources with my friend, I am able to help you guys. o w o 

Disclaimer:

I used Photoshop CC and the process is the same on any other version of Photoshop you have. 

Make sure to comment on the Airplane R&T Story page if it helped some of you who were oblivious to it. :D