Camera roll vs My photo stream

I was trying to move photos from my iPhone to my iMac. I Hooked up my iPhone to my iMac. 613 photos showed up to import. The 613 photos were in my camera roll. I have 974 photos in my iPhone photo stream. What is the difference between camera roll and My photo stream. Which one should I import in order to save the photos from my iPhone to my iMac and then transfer them to an external hard drive. Thanks for your input and help.