Photo Library-Photo Stream-Camera Roll , whats the difference between these Can someone please tell me the difference between Camera Roll, Photo Stream and Photo Library.  And why can't I delete anything in Photo Library. If I delete pictures from photo stream , it says it will erase them off my computer as well?
I want to keep all the pics on my computer but delete on my phone to free up space. How do I do that?  And is there a fast way to delete all pics or one by one... I have over 2000 pics! I have a iPhone 4S.
Camera Roll contains photos you have taken on your device, accessible locally. 

Photo Stream is a set of photos synced to all your devices, and only takes the last 1000 photos or from the past 30 days, whichever comes first. Photo Stream does not affect your iCloud storage. 

(iCloud) Photo Library is where photos can be uploaded to, outside of the photos you take on your iDevice (so DSLR photos can be in Photo Library) and it does affect and consume your iCloud storage.