Why do all pictures delete when I only want to delete on phone

Hello,

I 2 questions regarding storage, pictures and photostream.

I have an iPhone 5 16gb. i am having difficulty understanding my camera roll vs photostream. if I delete from camera roll it still remains in photostream. If I want to delete from photostream it appears that it will also delete on all my devices when

i only would like to delete on that one device. Why does this happen?

do I need to turn off icloud?

Thank you.

I Just recently downloaded Dropbox and carousel. My pictures appear there but again whenI delete from photostream it also disappears from Dropbox as well as carousel. Shouldnt Dropbox store all photos and keep them regardless of what I do in my photostream. Carousel as well?

uultimately I would

like to free up space ony phone without having to lose all pictures on all devices. how can this be saved?

Thank you again!

Julie